CI: prune unused Docker images after use

At the moment, we're running out of disk space when building packages in
Docker containers.  This is because our CI VMs have only 14 GB of
storage.  To help deal with this problem, let's add an option to the
run_dockers.bsh script to prune images after using them so we can free
some of that disk space and let the remaining jobs complete.

.github/workflows/ci.yml

@@ -98,4 +98,4 @@ jobs:
     - uses: actions/setup-ruby@v1
     - run: git clone https://github.com/git-lfs/build-dockers.git "$HOME/build-dockers"
     - run: (cd "$HOME/build-dockers" && ./build_dockers.bsh)
-    - run: DOCKER_AUTOPULL=0 ./docker/run_dockers.bsh
+    - run: DOCKER_AUTOPULL=0 ./docker/run_dockers.bsh --prune

.github/workflows/release.yml

@@ -70,7 +70,7 @@ jobs:
     - run: gem install packagecloud-ruby
     - run: git clone https://github.com/git-lfs/build-dockers.git "$HOME/build-dockers"
     - run: (cd "$HOME/build-dockers" && ./build_dockers.bsh)
-    - run: DOCKER_AUTOPULL=0 ./docker/run_dockers.bsh
+    - run: DOCKER_AUTOPULL=0 ./docker/run_dockers.bsh --prune
     # If this is a pre-release tag, don't upload anything to packagecloud.
     - run: '[ -z "${GITHUB_REF%%refs/tags/*-pre*}" ] || ./script/packagecloud.rb'
       env:

docker/run_dockers.bsh

@@ -42,8 +42,11 @@ function split_image_name()
 
 # Parse Arguments
 IMAGES=()
+PRUNE=
 while [[ $# > 0 ]]; do
-  if [ "$1" == "--" ]; then
+  if [ "$1" = "--prune" ]; then
+    PRUNE=t
+  elif [ "$1" == "--" ]; then
     shift
     DOCKER_CMD="${@}"
     break
@@ -108,6 +111,10 @@ for IMAGE_NAME in "${IMAGES[@]}"; do
                    -v "${MINGW_PATCH}${IMAGE_REPO_DIR}:/repo" \
                    gitlfs/build-dockers:${IMAGE_NAME} ${DOCKER_CMD-}
 
+  if [ -n "$PRUNE" ]
+  then
+    $SUDO docker rmi -f "gitlfs/build-dockers:${IMAGE_NAME}"
+  fi
 done
 
 echo "Docker run completed successfully!"