How to use this image

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 a Keycloak instance

To run Keycloak in production mode, you first need valid HTTPS certificates. Run the following commands to create a self-signed certificate. This creates tls.crt and tls.key and is valid for https://localhost⁠ only.

mkdir -p keycloak-certs
openssl req -x509 -newkey rsa:4096 -keyout keycloak-certs/tls.key -out keycloak-certs/tls.crt -days 365 -nodes -subj "/CN=localhost"

Next, run the following command to start the Keycloak instance. Replace <tag> with the image variant you want to run.

docker run -p 8443:8443 \
  -e KC_BOOTSTRAP_ADMIN_USERNAME=admin \
  -e KC_BOOTSTRAP_ADMIN_PASSWORD=admin \
  -e KC_HTTPS_CERTIFICATE_FILE=/etc/x509/https/tls.crt \
  -e KC_HTTPS_CERTIFICATE_KEY_FILE=/etc/x509/https/tls.key \
  -e KC_HOSTNAME=localhost \
  -v $(pwd)/keycloak-certs:/etc/x509/https \
  dhi.io/keycloak:<tag> start

Go to https://localhost:8443⁠ and login using admin for both the username and password.

Create an OIDC client app

This section walks you through creating a realm, adding a user, registering an OIDC client, and requesting an access token from Keycloak. Use the Keycloak instance you created above, running on https://localhost:8443⁠.

If you aren't already, login to https://localhost:8443/admin/⁠ using admin for username and password.

Create a Realm

Next you create a Realm, which is similar conceptually to an Azure Entra tenant. To create a Realm, select Manage Realms in the top left dropdown and then Create Realm.

Name it demo-realm and then click Create. In top left you should see demo-realm denoted as the Current realm you are now managing.

Add a User

In the left menu, go to Users and then Create new user. Enter docker-test-user for the Username field and then click Create.

Next set a password for the user. Go to the Credentials tab and click Set password. Enter a password and uncheck Temporary. Click Save to set the password.

Create an OIDC app

Next you create an OIDC client application. Go to ClientsCreate client. Ensure the Client Type field is set to OpenId Connect, then enter demo-client for the Client ID. Then click Next.

In the Capability config screen, toggle on Client authentication and then check the box for Service accounts roles which will allow authentication using client ID and secret. Click Next and then Save.

Now that you have a client application, go to the Credentials tab and copy the Client Secret. You use this below along with the client ID to authenticate.

Get an Access Token via REST API

Use the following curl command to get an access token from your client app. Replace <your-client-secret> with the secret copied from the previous step.

CLIENT_ID=demo-client
CLIENT_SECRET=<your-client-secret>
REALM=demo-realm

curl -k -s \
  -X POST "https://localhost:8443/realms/$REALM/protocol/openid-connect/token" \
  -H "Content-Type: application/x-www-form-urlencoded" \
  -d "grant_type=client_credentials" \
  -d "client_id=$CLIENT_ID" \
  -d "client_secret=$CLIENT_SECRET" \
  -d "scope=openid" | jq .

You should see output like the following, copy the access_token to use in the next step.

{
  "access_token": "<your-access-token>",
  "expires_in": 300,
  "refresh_expires_in": 0,
  "token_type": "Bearer",
  "id_token": "sample-id-token",
  "not-before-policy": 0,
  "scope": "openid profile email"
}

Verify the token

To verify the token works, make a call to the userinfo endpoint using the access token.

curl -k -H "Authorization: Bearer <your-access-token>" \
  "https://localhost:8443/realms/demo-realm/protocol/openid-connect/userinfo"

You should get a result like the following output which shows the subject ID.

{
   "sub":"070d5d56-cd19-4e88-b218-fe1c290a98ee",
   "email_verified":false,
   "preferred_username":"service-account-demo-client"
}

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
  • Do not include a shell or a package manager
  • Contain only the minimal set of libraries needed to run the app

• Build-time variants typically 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

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. For framework images, this is typically going to be an image tagged as dev because it has the tools needed to install packages and dependencies.

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.

Troubleshooting migration

The following are common issues that you may encounter during 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.

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.

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. To avoid issues, configure your application to listen on port 1025 or higher inside the container, even if you map it to a lower port on the host. For example, docker run -p 80:8080 my-image will work because the port inside the container is 8080, and docker run -p 80:81 my-image won't work because the port inside the container is 81.

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.