ci: split docker build workflow from test workflow.

Split up the Github workflow. Instead of one workflow with two jobs, one
to build the Docker image and one to test it, split it into two separate
workflows, one to build and one to test. This way if the Docker build
fails it doesn't try to run the tests, and if the tests fail it only
marks the test workflow as failed, not the entire workflow.

This is especially so the workflows page doesn't show everything as
failing just because the tests failed.

https://github.com/danbooru/danbooru/actions

.github/workflows/docker-build.yaml

@@ -0,0 +1,78 @@
+# On every commit, build and push a Docker image to the GitHub Container
+# registry and to DockerHub.
+#
+# The resulting Docker images are tagged with the full commit hash, the git
+# branch name, the git tag, and the 'latest' tag for the latest commit to
+# master.
+#
+# https://github.com/danbooru/danbooru/pkgs/container/danbooru
+# https://hub.docker.com/r/evazion/danbooru
+#
+# https://docs.github.com/en/packages/working-with-a-github-packages-registry/working-with-the-container-registry
+# https://docs.github.com/en/packages/managing-github-packages-using-github-actions-workflows/publishing-and-installing-a-package-with-github-actions
+# https://docs.github.com/en/actions/guides/publishing-docker-images
+
+name: Docker Build
+
+# https://docs.github.com/en/actions/reference/events-that-trigger-workflows
+on: [push, create]
+
+jobs:
+  docker-build:
+    runs-on: ubuntu-latest
+
+    steps:
+      # https://github.com/docker/setup-buildx-action
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v1
+
+      # https://github.com/docker/login-action
+      # https://github.com/docker/build-push-action/blob/master/docs/advanced/push-multi-registries.md
+      - name: Login to Github Container Registry
+        uses: docker/login-action@v1
+        with:
+          registry: ghcr.io
+          username: ${{ github.repository_owner }}
+          password: ${{ github.token }}
+
+      - name: Login to DockerHub
+        uses: docker/login-action@v1
+        with:
+          username: ${{ secrets.DOCKERHUB_USERNAME }}
+          password: ${{ secrets.DOCKERHUB_TOKEN }}
+
+      # Generate Docker tags from Git tags.
+      # https://github.com/docker/metadata-action
+      - name: Generate Docker tags
+        uses: docker/metadata-action@v3
+        id: metadata
+        with:
+          images: |
+            docker.io/evazion/danbooru
+            ghcr.io/danbooru/danbooru
+          tags: |
+            type=sha,format=short,prefix=
+            type=sha,format=long,prefix=
+            type=ref,event=branch
+            type=ref,event=tag
+            type=ref,event=pr
+          # Tag `latest` on every commit pushed to master
+          # https://github.com/docker/metadata-action/issues/112
+          flavor: |
+            latest=${{ github.ref == 'refs/heads/master' }}
+
+      # https://github.com/docker/build-push-action
+      - name: Build image
+        uses: docker/build-push-action@v2
+        with:
+          push: true
+          tags: ${{ steps.metadata.outputs.tags }}
+          labels: ${{ steps.metadata.outputs.labels }}
+
+          # https://github.com/docker/build-push-action/blob/master/docs/advanced/cache.md#registry-cache
+          # https://dev.to/pst418/speed-up-multi-stage-docker-builds-in-ci-cd-with-buildkit-s-registry-cache-11gi
+          cache-from: type=registry,ref=ghcr.io/${{ github.repository }}:buildcache
+          cache-to: type=registry,ref=ghcr.io/${{ github.repository }}:buildcache,mode=max
+
+          build-args: |
+            SOURCE_COMMIT=${{ github.sha }}