Add source label to Dockerfile

[BLACK4585]

The source label in the Docker container helps the Renovate bot correctly fetch and show changelogs for new versions fetched from Docker Hub.
This is described here:
https://docs.renovatebot.com/modules/datasource/docker/#description

This is useful when using GitOps, as I do.

[hollanbm]

@BLACK4585 @CyferShepard The reason the labels are missing is that the output from the docker-metadata step is not referenced during image build

Before

      - name: Build Docker image
        uses: docker/build-push-action@v5
        with:
          context: .
          push: true
          tags: |
            cyfershepard/jellystat:latest
            cyfershepard/jellystat:${{ env.VERSION }}
            ghcr.io/cyfershepard/jellystat:latest
            ghcr.io/cyfershepard/jellystat:${{ env.VERSION }}
          platforms: linux/amd64,linux/arm64,linux/arm/v7

AFTER

      - name: Build Docker image
        uses: docker/build-push-action@v5
        with:
          context: .
          push: true
          labels: ${{ steps.meta.outputs.labels }}
          tags: |
            ${{ steps.meta.outputs.tags }}
            cyfershepard/jellystat:latest
            cyfershepard/jellystat:${{ env.VERSION }}
            ghcr.io/cyfershepard/jellystat:latest
            ghcr.io/cyfershepard/jellystat:${{ env.VERSION }}
          platforms: linux/amd64,linux/arm64,linux/arm/v7