Fix #858: tell buildx which platform to build for

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.

While `depends_on` controls the runtime start order of containers, the
build-time dependency inference can be tricky in this parallel execution
environment.

One way to fix this is to explicitly tell `buildx` which platform to
build for. This forces buildx to build a single-platform image and load
its layers into the local image store, making it available to subsequent
build steps.

Author: mhucka

docker-compose.yml

@@ -1,11 +1,13 @@
 services:
     qsim:
+        platform: linux/amd64
         image: qsim
         container_name: qsim
         build:
             context: ./
             dockerfile: Dockerfile
     qsim-cxx-tests:
+        platform: linux/amd64
         image: qsim-cxx-tests
         container_name: qsim-cxx-tests
         build:
@@ -14,6 +16,7 @@ services:
         depends_on:
         - qsim
     qsim-py-tests:
+        platform: linux/amd64
         image: qsim-py-tests
         container_name: qsim-py-tests
         build: