ci: cancel superseded runs with native concurrency groups

cancel-redundant-workflows.yml had some overhead as it was spawning an extra
workflow_run job per workflow. With concurrency groups, a new push cancels the
PR's previous run directly.

A cancelled step can leave its docker container running against the shared
sstate.  Label containers with the runner name and remove them in the cleanup
step. Rename docker-prune-images to docker-cleanup accordingly.

Signed-off-by: Andrei Gherzan <andrei@gherzan.com>

.github/actions/docker-build/action.yml

@@ -26,7 +26,7 @@ runs:
       run: |
         # Images older than max_age_days are rebuilt so that base image and
         # package updates are picked up.
-        # Keep docker-prune-images at twice this.
+        # Keep docker-cleanup at twice this.
         max_age_days=7
         tag="$(git rev-parse --short=12 HEAD:.github/workflows/docker-images)"
         image="${{ inputs.docker_image }}:$tag"

.github/actions/docker-cleanup/action.yml

@@ -2,11 +2,20 @@
 #
 # SPDX-License-Identifier: MIT
 
-name: "Prune old CI docker images"
+name: "Cleanup CI docker containers and images"
 
 runs:
   using: "composite"
   steps:
+    - name: Remove CI containers left on this runner
+      shell: bash
+      # A runner executes one job at a time, so any container labelled with
+      # its name is left over from this job (e.g. after a cancellation) or a
+      # previous one.
+      run: |
+        echo "Removing CI containers left by runner $RUNNER_NAME..."
+        docker ps -aq --filter "label=meta-raspberrypi-ci.runner=$RUNNER_NAME" |
+          xargs -r docker rm -f
     - name: Prune old CI docker images
       shell: bash
       # Only images built by docker-build carry the label. Keep this at twice