github: Add a manual workflow for building and pushing docker images

.github/workflows/docker.yml

@@ -0,0 +1,71 @@
+name: Build docker images
+on:
+  workflow_dispatch:
+    inputs:
+      commit:
+        description: 'Commit to extract from'
+        type: string
+      branch:
+        description: 'Branch to extract from'
+        type: string
+      login:
+        description: 'Log in to Docker Hub'
+        type: boolean
+        default: true
+      push:
+        description: 'Push the built images'
+        type: boolean
+        default: false
+
+jobs:
+  prepare:
+    runs-on: ubuntu-latest
+    outputs:
+      TAG: ${{steps.get-parameters.outputs.TAG}}
+    steps:
+      - name: Download artifacts
+        uses: dawidd6/action-download-artifact@v2
+        with:
+          workflow: build.yml
+          workflow_conclusion: success
+          commit: ${{inputs.commit}}
+          branch: ${{inputs.branch}}
+          event: push
+          name: parameters
+      - name: Fetch build variables
+        id: get-parameters
+        run: |
+          cat parameters.txt >> $GITHUB_OUTPUT
+          cat $GITHUB_OUTPUT
+
+  docker-build:
+    needs: [prepare]
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - { file: Dockerfile,     maintag: latest,  prefix: }
+          - { file: Dockerfile.dev, maintag: dev,     prefix: dev- }
+    steps:
+      - uses: actions/checkout@v3
+        with:
+          ref: ${{ (inputs.commit != '' && inputs.commit) || inputs.branch }}
+      - name: Log in to Docker Hub
+        if: ${{inputs.login}}
+        uses: docker/login-action@v2
+        with:
+          username: ${{secrets.DOCKER_USERNAME}}
+          password: ${{secrets.DOCKER_PASSWORD}}
+      - name: Build Docker images
+        uses: docker/build-push-action@v4
+        with:
+          context: .
+          push: ${{inputs.push}}
+          file: ./${{matrix.file}}
+          tags: |
+            mstorsjo/llvm-mingw:${{matrix.maintag}}
+            mstorsjo/llvm-mingw:${{matrix.prefix}}${{needs.prepare.outputs.TAG}}
+      - name: Inspect Docker images
+        run: |
+          docker images