Preinstall collections and fix docker push on PRs

Pushes to tag matching branch name if not on the main branch, but will use latest in the specific case of a pipeline run on mainline.

Pre-installs the three ansible-galaxy collections used in the main ansible-repo, can also be extended later.

.gitlab-ci.yml

@@ -11,11 +11,15 @@ services:
 build:
   stage: build
   script:
+    - export TAG_NAME=$CI_COMMIT_BRANCH
+    - if [[ "$CI_COMMIT_BRANCH" == "$CI_DEFAULT_BRANCH" ]]; then
+        export TAG_NAME="latest";
+      fi 
     - docker login -u $CI_REGISTRY_USER -p $CI_REGISTRY_PASSWORD $CI_REGISTRY
     - docker build --pull --build-arg BUILDKIT_INLINE_CACHE=1
-      --cache-from $CI_REGISTRY_IMAGE:latest
+      --cache-from $CI_REGISTRY_IMAGE:$TAG_NAME
       --tag $CI_REGISTRY_IMAGE:$CI_COMMIT_SHA
-      --tag $CI_REGISTRY_IMAGE:latest
+      --tag $CI_REGISTRY_IMAGE:$TAG_NAME
       "."
     - docker push $CI_REGISTRY_IMAGE:$CI_COMMIT_SHA
-    - docker push $CI_REGISTRY_IMAGE:latest
+    - docker push $CI_REGISTRY_IMAGE:$TAG_NAME