Add org.opencontainers.image.source to Dockerfiles (#9211)

### Purpose

The OCI image spec specifies well-defined
[annotations](https://github.com/opencontainers/image-spec/blob/main/annotations.md)
that can be added to images.
Theses annotations can then be used by other tools to gather more
information of an image.

This PR adds the `org.opencontainers.image.source` to allow tools such
as [renovate](https://github.com/renovatebot/renovate) to find the
release notes of a give version.

~~I've only done this change for `Dockerfile`. Should I also add the
label to the other dockerfiles?~~
I've now added the source annotations to all `Dockerfile`s & action
workflows.

### Testing

None, change was done by following the [renovate
documentation](https://docs.renovatebot.com/modules/datasource/docker/).

.github/workflows/build-syncthing.yaml

@@ -808,6 +808,7 @@ jobs:
             tags=${{ matrix.image }}:edge
           fi
           echo "DOCKER_TAGS=$tags" >> $GITHUB_ENV
+          echo "VERSION=$version" >> $GITHUB_ENV
 
       - name: Build and push Docker image
         uses: docker/build-push-action@v5
@@ -817,6 +818,9 @@ jobs:
           platforms: linux/amd64,linux/arm64,linux/arm/7
           push: ${{ env.DOCKER_PUSH == 'true' }}
           tags: ${{ env.DOCKER_TAGS }}
+          outputs: |
+            annotation.org.opencontainers.image.version="${{ env.VERSION }}"
+            annotation.org.opencontainers.image.revision="${{ github.sha }}"
 
   #
   # Check for known vulnerabilities in Go dependencies