Fix #858: tell buildx which platform to build for

[mhucka]

Docker builds on GitHub are failing with the problem that Docker build is trying to get qsim images from docker.io:

#1 [internal] load local bake definitions
#1 reading from stdin 935B done
#1 DONE 0.0s

#2 [qsim internal] load build definition from Dockerfile
#2 transferring dockerfile: 1.34kB done
#2 DONE 0.0s

#3 [qsim-py-tests internal] load build definition from Dockerfile
#3 transferring dockerfile: 381B done
#3 DONE 0.0s

#4 [qsim-cxx-tests internal] load build definition from Dockerfile
#4 transferring dockerfile: 208B done
#4 DONE 0.0s

#5 [qsim-cxx-tests internal] load metadata for #docker.io/library/qsim:latest

Apparently, this happens because the buildx plugin, which is the default builder in many modern Docker setups, can build images in parallel and uses an isolated builder environment. An image built for one service (like qsim): might not be immediately loaded into the local Docker daemon's image store. When the build for a dependent service (like qsim-cxx-tests) starts, it can't find the qsim base image locally and falls back to searching Docker Hub.

This is despite the fact that a plain docker compose build locally (at least in my Debian Linux environment) works as expected.

After a lot of trial and error, the only solution I could find is to split up the docker compose build … into 2 steps, the first being to build the qsim base image separately:

- name: Build Docker images
  run: |
    docker compose build qsim-base-image
    docker compose build qsim-cxx-tests-image qsim-py-tests-image

In addition to this change, in the process of debugging this I found it useful to make some housekeeping changes to the Dockerfile and docker-compose.yml files, in particular to change the names of jobs and images to distinguish the different things being run or built. (Having several things all named qsim was not helpful.)

Noting here what else I tried and that didn't work:

• What I thought would be the obvious solution (i.e., using the command-line argument --parallel 1 or setting the env var COMPOSE_PARALLEL_LIMIT to tell buildx not to build in parallel) did not work on GitHub. Since I can't reproduce the problem locally, I can't be sure if that would have solved it anyway.

• Trying to use the buildx bake command first, like this:

  docker buildx bake --load --progress=plain -f docker-compose.yml
  docker compose build

• Trying to clear the Docker cache.