Advanced backend management

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Backend access to host ports

Docker Assemble requires its own buildkit instance to be running in a Docker container on the local system. You can start and manage the backend using the backend subcommand of docker assemble. For more information, see Install Docker Assemble.

As the backend runs in a container with its own network namespace, it cannot access host resources directly. This is most noticeable when trying to push to a local registry as localhost:5000.

The backend supports a sidecar container which proxies ports from within the backend container to the container’s gateway (which is in effect a host IP). This is sufficient to allow access to host ports which have been bound to (or to the gateway specifically), but not ones which are bound to

By default, port 5000 is proxied in this way, as that is the most common port used for a local registry to allow access to a local registry on localhost:5000 (the most common setup). You can proxy other ports using the --allow-host-port option to docker assemble backend start.

For example, to expose port 6000 instead of port 5000, run:

$ docker assemble backend start --allow-host-port 6000


  • You can repeat the --allow-host-port option or give it a comma separated list of ports.
  • Passing --allow-host-port 0 disables the default and no ports are exposed. For example:

    $ docker assemble backend start --allow-host-port 0

  • On Docker Desktop, this functionality allows the backend to access ports on the Docker Desktop VM host, rather than the Windows or macOS host. To access the the Windows or macOS host port, you can use host.docker.internal as usual.

Backend sub-commands


The info sub-command describes the backend:

~$ docker assemble backend info
ID: 2f03e7d288e6bea770a2acba4c8c918732aefcd1946c94c918e8a54792e4540f (running)
Image: docker/assemble-backend@sha256:«…»

Sidecar containers:
 - 0f339c0cc8d7 docker-assemble-backend-username-proxy-port-5000 (running)

Found 1 worker(s):

 - 70it95b8x171u5g9jbixkscz9
    - linux/amd64
    - com.docker.assemble.commit: «…»
    - org.mobyproject.buildkit.worker.executor: oci
    - org.mobyproject.buildkit.worker.hostname: 2f03e7d288e6
    - org.mobyproject.buildkit.worker.snapshotter: overlayfs

Build cache contains 54 entries, total size 3.65GB (0B currently in use)


The stop sub-command destroys the backend container

~$ docker assemble backend stop


The logs sub-command displays the backend logs.

~$ docker assemble backend logs


The build cache gets lost when the backend is stopped. To avoid this, you can create a volume named docker-assemble-backend-cache-«username» and it will automatically be used as the build cache.

Alternatively you can specify a named docker volume to use for the cache. For example:

~$ docker volume create $USER-assemble-cache
~$ docker assemble backend start --cache-volume=username-assemble-cache
Pulling image «…»: Success
Started container "docker-assemble-backend-username" (74476d3fdea7)

For information regarding the current cache contents, run the command docker assemble backend cache.

To clean the cache, rundocker assemble backend cache purge.

Backend, Assemble, Docker Enterprise, plugin, Spring Boot, .NET, c#, F#