ci: Create dev-builds on push to develop branch

.github/workflows/dev-build.yml

+name: Create Dev-Build
+
+on:
+  push:
+    branches:
+      - develop
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      # DOCKER RELATED
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v1
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v1
+      - name: Login to GitHub Container Registry
+        uses: docker/login-action@v1
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+      - name: Build and push
+        uses: docker/build-push-action@v2
+        with:
+          context: .
+          file: ./Dockerfile.develop
+          platforms: linux/amd64,linux/arm/v7,linux/arm64
+          push: true
+          tags: |
+            ghcr.io/griefed/ltt-mapmaker:develop
+            ghcr.io/griefed/ltt-mapmaker:develop-${{ github.sha }}