Build Docker Hub images via Github Actions

Since docker hub stopped Autobuild, we need to switch to something else.
This will trigger building of images on Github Actions and pushes them
to Docker Hub.

You only need to add 3 secrets before you merge this PR to have it working directly.

- DOCKERHUB_USERNAME : The username of the account you are going to push the builds to
- DOCKERHUB_TOKEN : The token needed to login and push builds
- DOCKERHUB_REPO : The repo name in the following form `index.docker.io/<user>/<repo>`
  So for vaultwarden that would be `index.docker.io/vaultwarden/server`

Also some small modifications to the other workflows.

hooks/push

@@ -25,7 +25,10 @@ echo ">>> Starting local Docker registry..."
 # Use host networking so the buildx container can access the registry via
 # localhost.
 #
-docker run -d --name registry --network host registry:2 # defaults to port 5000
+# First check if there already is a registry container running, else skip it.
+# This will only happen either locally or running it via Github Actions
+#
+docker start registry > /dev/null 2>&1 || docker run -d --name registry --network host registry:2 # defaults to port 5000
 
 # Docker Hub sets a `DOCKER_REPO` env var with the format `index.docker.io/user/repo`.
 # Strip the registry portion to construct a local repo path for use in `Dockerfile.buildx`.
@@ -49,7 +52,12 @@ echo ">>> Setting up Docker Buildx..."
 #
 # Ref: https://github.com/docker/buildx/issues/94#issuecomment-534367714
 #
-docker buildx create --name builder --use --driver-opt network=host
+# Check if there already is a builder running, else skip this and use the existing.
+# This will only happen either locally or running it via Github Actions
+#
+if ! docker buildx inspect builder > /dev/null 2>&1 ; then
+    docker buildx create --name builder --use --driver-opt network=host
+fi
 
 echo ">>> Running Docker Buildx..."