contrib: push Docker build-container to GitHub registry (#2971)

This adds a workflow to push the build-container described in Gluons
Dockerfile to the freifunk-gluon organizations container registry.

This will create a image

 - For every push to master
 - For every tagged version
 - For every PR against master

Signed-off-by: David Bauer <mail@david-bauer.net>

.github/workflows/build-container.yml

+# Based on the example from https://docs.github.com/en/actions/publishing-packages/publishing-docker-images
+name: Create and publish a Docker image
+
+on:
+  push:
+    branches:
+      - 'master'
+    tags:
+      - 'v*'
+  pull_request:
+    branches:
+      - 'master'
+
+env:
+  REGISTRY: ghcr.io
+  IMAGE_NAME: gluon-build
+
+jobs:
+  build-and-push-image:
+    runs-on: ubuntu-latest
+    permissions: write-all
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v3
+      - name: Log in to the Container registry
+        uses: docker/login-action@65b78e6e13532edd9afa3aa52ac7964289d1a9c1
+        with:
+          registry: ${{ env.REGISTRY }}
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+      - name: Extract metadata (tags, labels) for Docker
+        id: meta
+        uses: docker/metadata-action@9ec57ed1fcdbf14dcef7dfbe97b2010124a938b7
+        with:
+          images: ${{ env.REGISTRY }}/freifunk-gluon/${{ env.IMAGE_NAME }}
+      - name: Build and push Docker image
+        uses: docker/build-push-action@f2a1d5e99d037542a71f64918e516c093c6f3fc4
+        with:
+          context: ./contrib/docker
+          push: true
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}