Prerequisites

All examples in this guide use the public image. If you've mirrored the repository for your own use (for example, to your Docker Hub namespace), update your commands to reference the mirrored image instead of the public one.

For example:

• Public image: dhi.io/<repository>:<tag>
• Mirrored image: <your-namespace>/dhi-<repository>:<tag>

For the examples, you must first use docker login dhi.io to authenticate to the registry to pull the images.

Start an Alertmanager instance

The Docker Hardened Alertmanager image includes a default configuration file and ships with sensible default CMD flags (--config.file=/etc/alertmanager/alertmanager.yml --storage.path=/alertmanager), so you can start Alertmanager with a single command and no mounted files. The web UI and API are served on port 9093.

Run the following command and replace <tag> with the image variant you want to run.

$ docker run -d --name alertmanager -p 9093:9093 \
  dhi.io/alertmanager:<tag>

Verify it's running:

$ curl http://localhost:9093/-/ready
OK

Common Alertmanager use cases

Mount a custom configuration

Override the bundled default config by mounting your own YAML file to /etc/alertmanager/alertmanager.yml.

$ docker run -d --name alertmanager -p 9093:9093 \
  -v /path/to/config.yml:/etc/alertmanager/alertmanager.yml:ro \
  dhi.io/alertmanager:<tag>

A minimal config that routes all alerts to a webhook receiver:

route:
  receiver: 'webhook'
receivers:
  - name: 'webhook'
    webhook_configs:
      - url: 'http://example-receiver.local:5001/'

Persist Alertmanager state

Alertmanager stores silence snapshots and notification log state on disk at --storage.path. The image pre-creates the default path /alertmanager with ownership 65532:65532, which matches the nonroot user the container runs as. Mount a named volume to this path and no additional setup is required:

$ docker run -d --name alertmanager -p 9093:9093 \
  -v alertmanager_data:/alertmanager \
  dhi.io/alertmanager:<tag>

If you mount a volume to a different path, Docker auto-creates the target as root:root, which the nonroot user (UID 65532) cannot write to. Alertmanager will start and serve requests, but will silently fail to persist state and log permission denied errors on shutdown. Pre-chown the volume before starting Alertmanager:

$ docker volume create alertmanager_data
$ docker run --rm -v alertmanager_data:/data --user 0 \
    dhi.io/busybox:<tag> chown 65532:65532 /data
$ docker run -d --name alertmanager -p 9093:9093 \
  -v alertmanager_data:/data \
  dhi.io/alertmanager:<tag> \
  --config.file=/etc/alertmanager/alertmanager.yml \
  --storage.path=/data

For host bind-mounts, run chown 65532:65532 on the host directory before starting the container.

Run with Prometheus using Docker Compose

Run Prometheus and Alertmanager together using Docker Compose. Prometheus reaches Alertmanager by service name on the Compose network.

services:
  prometheus:
    image: prom/prometheus:latest
    volumes:
      - ./prometheus.yml:/etc/prometheus/prometheus.yml:ro
    ports:
      - 9090:9090
  alertmanager:
    image: dhi.io/alertmanager:<tag>
    ports:
      - 9093:9093
    volumes:
      - ./alertmanager/config.yml:/etc/alertmanager/alertmanager.yml:ro
      - alertmanager_data:/alertmanager

volumes:
  alertmanager_data: {}

In prometheus.yml, point Prometheus at Alertmanager:

alerting:
  alertmanagers:
    - static_configs:
        - targets: ['alertmanager:9093']

Non-hardened images vs Docker Hardened Images

Key differences

Why no shell or package manager?

Docker Hardened Images prioritize security through minimalism:

• Reduced attack surface: Fewer binaries mean fewer potential vulnerabilities
• Immutable infrastructure: Runtime containers shouldn't be modified after deployment
• Compliance ready: Meets strict security requirements for regulated environments

The hardened images intended for runtime don't contain a shell nor any tools for debugging. Common debugging methods for applications built with Docker Hardened Images include:

• Docker Debug to attach to containers
• Docker's Image Mount feature to mount debugging tools
• Ecosystem-specific debugging approaches

Docker Debug provides a shell, common debugging tools, and lets you install other tools in an ephemeral, writable layer that only exists during the debugging session.

For example, you can use Docker Debug:

$ docker debug alertmanager

Or mount debugging tools with the Image Mount feature:

$ docker run --rm -it --pid container:alertmanager \
  --mount=type=image,source=dhi.io/busybox,destination=/dbg,ro \
  dhi.io/alertmanager:<tag> /dbg/bin/sh

For Alertmanager specifically, most operational inspection can also be done through the HTTP API without a shell: /-/ready, /-/healthy, /api/v2/status, /api/v2/alerts, /api/v2/silences, and /metrics.

Image variants

Docker Hardened Images come in different variants depending on their intended use.

Runtime variants are designed to run your application in production. These images are intended to be used either directly or as the FROM image in the final stage of a multi-stage build. These images typically:

• Run as the nonroot user (UID 65532)
• Do not include a shell or a package manager
• Contain only the minimal set of libraries needed to run the app

Build-time variants include dev in the variant name and are intended for use in the first stage of a multi-stage Dockerfile. These images typically:

• Run as the root user
• Include a shell and package manager
• Are used to build or compile applications

The Alertmanager image is published in the following variant combinations:

Alpine-based equivalents are also published with tag patterns <version>-alpine3.23, <version>-alpine3.23-fips, etc.

To view all published tags and get more information about each variant, select the Tags tab for this repository and select a tag.

FIPS variants

FIPS variants include fips in the variant name and tag. They come in both runtime and build-time variants. These variants use cryptographic modules that have been validated under FIPS 140, a U.S. government standard for secure cryptographic operations.

FIPS variants of the Alertmanager image are drop-in replacements for the standard runtime variant — they have the same CMD, working directory, and nonroot user (UID 65532). No changes are required to the Alertmanager binary or config file to run under FIPS mode.

Pull a FIPS variant

$ docker pull dhi.io/alertmanager:<version>-fips

Verify the FIPS attestation

FIPS variants include a signed FIPS attestation listing the cryptographic modules in the image and their validation status. Retrieve it with Docker Scout:

$ docker scout attest get \
    --predicate-type https://docker.com/dhi/fips/v0.1 \
    --predicate \
    dhi.io/alertmanager:<version>-fips

Example attestation output:

[
  {
    "name": "OpenSSL FIPS Provider",
    "package": "pkg:dhi/[email protected]",
    "standard": "FIPS 140-3",
    "status": "active",
    "certification": "CMVP #4985",
    "certificationUrl": "https://csrc.nist.gov/projects/cryptographic-module-validation-program/certificate/4985",
    "sunsetDate": "2030-03-10",
    "version": "3.1.2"
  },
  {
    "name": "Go Cryptographic Module",
    "package": "pkg:golang/golang.org/x/[email protected]#crypto/internal/fips140",
    "standard": "FIPS 140-3",
    "status": "in process",
    "version": "1.0.0"
  }
]

Runtime requirements specific to FIPS

Alertmanager itself does not require any FIPS-specific configuration flags. TLS and webhook cryptographic operations automatically use the FIPS-validated providers when the FIPS variant is running.

If you use Alertmanager integrations that depend on non-approved cryptographic algorithms (for example, webhook endpoints requiring MD5 or 3DES), those algorithms are disabled in FIPS mode and will fail at runtime. Review your receiver configurations for custom CA certificates signed with deprecated algorithms, tls_config blocks pinning non-approved cipher suites, and webhook receivers requiring older TLS versions.

What changes in FIPS mode

Compared to the standard runtime variant:

• Cryptographic modules: OpenSSL FIPS Provider 3.1.2 (CMVP #4985, active) and the Go FIPS 140-3 crypto module are used for all cryptographic operations.
• Available algorithms: only FIPS 140-approved algorithms are available. Non-approved algorithms fail at runtime rather than silently falling back.
• Image size: slightly larger due to the FIPS provider module.
• Compliance labels: carries com.docker.dhi.compliance=fips,stig,cis.
• Everything else: identical. Same Alertmanager version, same API, same config format, same nonroot UID 65532, same default CMD.

FIPS variants are appropriate for regulated environments such as FedRAMP, government, healthcare, financial services, and defense deployments.

Migrate to a Docker Hardened Image

To migrate your application to a Docker Hardened Image, you must update your Dockerfile. At minimum, you must update the base image in your existing Dockerfile to a Docker Hardened Image. This and a few other common changes are listed in the following table of migration notes:

The following steps outline the general migration process.

1. Find hardened images for your app.

   A hardened image may have several variants. Inspect the image tags and find the image variant that meets your needs.

2. Update the base image in your Dockerfile.

   Update the base image in your application's Dockerfile to the hardened image you found in the previous step.

3. For multi-stage Dockerfiles, update the runtime image in your Dockerfile.

   To ensure that your final image is as minimal as possible, you should use a multi-stage build. All stages in your Dockerfile should use a hardened image. While intermediary stages will typically use images tagged as dev, your final runtime stage should use a non-dev image variant.

4. Install additional packages.

   Docker Hardened Images contain minimal packages in order to reduce the potential attack surface. You may need to install additional packages in your Dockerfile. Inspect the image variants to identify which packages are already installed.

   Only images tagged as dev typically have package managers. You should use a multi-stage Dockerfile to install the packages. Install the packages in the build stage that uses a dev image. Then, if needed, copy any necessary artifacts to the runtime stage that uses a non-dev image.

   For Alpine-based images, you can use apk to install packages. For Debian-based images, you can use apt-get to install packages.

Troubleshoot migration

General debugging

The hardened images intended for runtime don't contain a shell nor any tools for debugging. The recommended method for debugging applications built with Docker Hardened Images is to use Docker Debug to attach to these containers. Docker Debug provides a shell, common debugging tools, and lets you install other tools in an ephemeral, writable layer that only exists during the debugging session.

For Alertmanager specifically, most operational debugging can be done through the HTTP API without a shell:

• GET /-/ready — readiness check
• GET /-/healthy — health check
• GET /api/v2/status — cluster and config status
• GET /api/v2/alerts — current alerts
• GET /api/v2/silences — current silences
• GET /metrics — Prometheus metrics

Permissions

By default image variants intended for runtime, run as the nonroot user. Ensure that necessary files and directories are accessible to the nonroot user. You may need to copy files to different directories or change permissions so your application running as the nonroot user can access them.

The most common permission issue with Alertmanager is mounting a volume to a custom storage path. Docker auto-creates bind-mount and volume targets as root:root, which UID 65532 cannot write to. Alertmanager will start and appear healthy but will silently fail to persist state, logging on shutdown:

level=ERROR msg="Creating shutdown snapshot failed"
  err="open /data/nflog.xxxxxxxx: permission denied"

Mount volumes to the default /alertmanager path (pre-created with correct ownership), or pre-chown custom paths to 65532:65532 before starting the container.

Privileged ports

Non-dev hardened images run as a nonroot user by default. As a result, applications in these images can't bind to privileged ports (below 1024) when running in Kubernetes or in Docker Engine versions older than 20.10. Alertmanager's default port 9093 is above 1024 and is unaffected.

No shell

By default, image variants intended for runtime don't contain a shell. Use dev images in build stages to run shell commands and then copy any necessary artifacts into the runtime stage. In addition, use Docker Debug to debug containers with no shell.

Entry point

Docker Hardened Images may have different entry points than images such as Docker Official Images. Use docker inspect to inspect entry points for Docker Hardened Images and update your Dockerfile if necessary. The Alertmanager image entry point is the alertmanager binary with default CMD --config.file=/etc/alertmanager/alertmanager.yml --storage.path=/alertmanager. To invoke the bundled amtool CLI, override the entry point with --entrypoint amtool.