[github] add workflow to build and push docker images

heavily inspired by and adapted from
https://github.com/danbooru/danbooru/blob/master/.github/workflows/docker-build.yaml

.github/workflows/docker.yml

@@ -0,0 +1,61 @@
+name: docker
+
+on:
+  push:
+    branches:
+    - master
+    tags:
+    - v[0-9]+.[0-9]+.[0-9]+
+
+permissions:
+  packages: write
+
+jobs:
+  docker:
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v4
+
+    # https://github.com/docker/setup-buildx-action
+    - name: Set up Docker Buildx
+      uses: docker/setup-buildx-action@v3
+
+    # https://github.com/docker/login-action
+    - name: Login to GitHub Container Registry
+      uses: docker/login-action@v3
+      with:
+        registry: ghcr.io
+        username: ${{ github.repository_owner }}
+        password: ${{ github.token }}
+
+    - name: Login to DockerHub
+      uses: docker/login-action@v3
+      with:
+        username: ${{ secrets.DOCKERHUB_USERNAME }}
+        password: ${{ secrets.DOCKERHUB_TOKEN }}
+
+    # https://github.com/docker/metadata-action
+    - name: Generate Docker tags
+      uses: docker/metadata-action@v5
+      id: metadata
+      with:
+        images: |
+          mikf123/gallery-dl
+          ghcr.io/mikf/gallery-dl
+        tags: |
+          type=sha,format=long,prefix=
+          type=ref,event=tag
+        # https://github.com/docker/metadata-action/issues/112
+        flavor: |
+          latest=${{ github.ref == 'refs/heads/master' }}
+
+    # https://github.com/docker/build-push-action
+    - name: Build image
+      uses: docker/build-push-action@v5
+      with:
+        push: true
+        tags: ${{ steps.metadata.outputs.tags }}
+        labels: ${{ steps.metadata.outputs.labels }}
+        platforms: linux/amd64