CI builds and pushes to Docker Hub. (#20)

.github/workflows/docker-build-and-push.yml

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+# Based on:
+#   https://docs.github.com/en/actions/publishing-packages/publishing-docker-images
+
+# This workflow uses actions that are not certified by GitHub.
+# They are provided by a third-party and are governed by
+# separate terms of service, privacy policy, and support
+# documentation.
+
+# GitHub recommends pinning actions to a commit SHA.
+# To get a newer version, you will need to update the SHA.
+# You can also reference a tag or branch, but the action may change without warning.
+
+name: Docker build and push
+
+on:
+  workflow_dispatch:
+  push:
+    branches:
+      - 'main'
+    tags:
+      - 'v*.*.*'
+  pull_request:
+    branches:
+      - 'main'
+
+env:
+  DOCKER_REPO: hltcoe/concrete-python
+  TEST_TAG: hltcoe/concrete-python:ci-test
+
+jobs:
+  push_to_registry:
+    name: Build and push Docker image
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check out the repo
+        uses: actions/checkout@v3
+
+      - name: Log in to Docker Hub
+        if: github.event_name != 'pull_request'
+        uses: docker/login-action@f4ef78c080cd8ba55a85445d5b36e214a81df20a
+        with:
+          username: ${{ secrets.DOCKER_USERNAME }}
+          password: ${{ secrets.DOCKER_PASSWORD }}
+
+      - name: Extract metadata (tags, labels) for Docker
+        id: meta
+        uses: docker/metadata-action@35e9aff4f5d665b5aa8a8f2adffaf8a1b5f49cc0
+        with:
+          images: ${{ env.DOCKER_REPO }}
+          tags: |
+            type=schedule
+            type=ref,event=branch
+            type=ref,event=tag
+            type=ref,event=pr
+            # set latest tag for default branch
+            type=raw,value=latest,enable={{is_default_branch}},priority=2000
+
+      - name: Build and export to Docker
+        uses: docker/build-push-action@3b5e8027fcad23fda98b2e3ac259d8d67585f671
+        with:
+          context: .
+          load: true
+          tags: ${{ env.TEST_TAG }}
+
+      - name: Test Docker image
+        run: |
+          docker run --rm ${{ env.TEST_TAG }} tox -e pep8,py
+      
+      - name: Push Docker image
+        uses: docker/build-push-action@3b5e8027fcad23fda98b2e3ac259d8d67585f671
+        with:
+          context: .
+          push: ${{ github.event_name != 'pull_request' }}
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}