ci: Use multiline chomp in YAML

It put the backslashes right into the bash command bash didn't really like that.
Bash escaped spaces and that lead to a bad command

.dockerignore

@@ -0,0 +1,8 @@
+.idea
+.git
+.gitlab-ci.yml
+.vscode
+
+# CI cache folder storing docker images
+ci-exports
+

.gitlab-ci.yml

@@ -1,5 +1,9 @@
 image: docker:19.03.12
 
+stages:
+  - docker_test
+  - docker_push
+
 variables:
   # When using dind service, we need to instruct docker to talk with
   # the daemon started inside of the service. The daemon is available
@@ -22,17 +26,81 @@ variables:
 services:
   - docker:19.03.12-dind
 
+.docker_cache:
+  cache:
+    # The same key should be used across branches
+    key: "$CI_COMMIT_REF_SLUG"
+    paths:
+      - ci-exports/*.tar
 
-before_script:
+# Make sure we can build a docker image
-  - docker login -u $CI_REGISTRY_USER -p $CI_REGISTRY_PASSWORD $CI_REGISTRY
+# It's cached for later jobs
-
+build_docker:
-build:
+  extends:
-  stage: build
+    - .docker_cache
+  stage: docker_test
   script:
-    - docker pull $CI_REGISTRY_IMAGE:latest || true
+    # Try to load latest branch image from local tar or from registry
-    - docker build --cache-from $CI_REGISTRY_IMAGE:latest --tag $CI_REGISTRY_IMAGE:$CI_COMMIT_TAG --tag $CI_REGISTRY_IMAGE:latest .
+    - docker load ci-exports/$CI_COMMIT_REF_SLUG.tar || docker pull $CI_REGISTRY_IMAGE:latest || true
+    - docker build --cache-from $CI_REGISTRY_IMAGE:latest --tag $CI_REGISTRY_IMAGE:latest .
+    - mkdir -p ci-exports/
+    - docker save $CI_REGISTRY_IMAGE:latest > ci-exports/$CI_COMMIT_REF_SLUG.tar
+
+# Publishes the configured CI registry (by default that's gitlab's registry)
+push_ci_registry:
+  extends:
+    - .docker_cache
+  stage: docker_push
+  cache:
+    policy: pull
+  before_script:
+    - docker login -u $CI_REGISTRY_USER -p $CI_REGISTRY_PASSWORD $CI_REGISTRY
+  script:
+    - cat ci-exports/$CI_COMMIT_REF_SLUG.tar | docker load
+    - docker tag $CI_REGISTRY_IMAGE:latest $CI_REGISTRY_IMAGE:$CI_COMMIT_TAG
     - docker push $CI_REGISTRY_IMAGE:$CI_COMMIT_TAG
     - docker push $CI_REGISTRY_IMAGE:latest
-
   only:
-    - tags
+    refs:
+      # Make sure to protect these tags!
+      - /^v(\d+\.){2,3}\d+$/
+      - /.+-release$/
+    variables:
+      - $CI_REGISTRY
+      - $CI_REGISTRY_USER
+      - $CI_REGISTRY_PASSWORD
+      - $CI_REGISTRY_IMAGE
+
+# Publishes the cached image to docker
+push_dockerhub_registry:
+  extends:
+    - .docker_cache
+  stage: docker_push
+  cache:
+    policy: pull
+  before_script:
+    - docker login -u $DOCKERHUB_REGISTRY_USER -p $DOCKERHUB_REGISTRY_PASSWORD $DOCKERHUB_REGISTRY
+  script:
+    - cat ci-exports/$CI_COMMIT_REF_SLUG.tar | docker load
+    - docker tag $CI_REGISTRY_IMAGE:latest $DOCKERHUB_REGISTRY_IMAGE:$CI_COMMIT_TAG
+    - docker tag $CI_REGISTRY_IMAGE:latest $DOCKERHUB_REGISTRY_IMAGE:latest
+    - docker push $DOCKERHUB_REGISTRY_IMAGE:$CI_COMMIT_TAG
+    - docker push $DOCKERHUB_REGISTRY_IMAGE:latest
+    # Push the readme to dockerhub
+    - >-
+      docker run -v $PWD:/workspace
+      -e DOCKERHUB_USERNAME="$DOCKERHUB_REGISTRY_USER"
+      -e DOCKERHUB_PASSWORD="$DOCKERHUB_REGISTRY_PASSWORD"
+      -e DOCKERHUB_REPOSITORY="$DOCKERHUB_REGISTRY_IMAGE"
+      -e README_FILEPATH='/workspace/README.md'
+      peterevans/dockerhub-description:2
+  only:
+    refs:
+      # Make sure to protect these tags!
+      - /^v(\d+\.){2,3}\d+$/
+      - /.+-release$/
+    variables:
+      - $DOCKERHUB_REGISTRY
+      - $DOCKERHUB_REGISTRY_USER
+      - $DOCKERHUB_REGISTRY_PASSWORD
+      - $DOCKERHUB_REGISTRY_IMAGE