Build `main` Docker images on every push to `main` branch (#272)

This makes it possible to test latest `carton` Docker builds from the `main` branch.

* Attempt to push Docker image in `swift.yml`

* Update swift.yml

* Update swift.yml

* Push only a single tag

* Use secrets.PAT for GHCR login

* Move nightly tags to separate `docker.yml` workflow

* Rename `nightly` to `main`

.github/workflows/docker.yml

@@ -0,0 +1,29 @@
+name: Build main Docker image
+
+on:
+  push:
+    branches: [main]
+
+jobs:
+  docker:
+    name: Push Docker image to GitHub Packages
+    runs-on: ubuntu-20.04
+    steps:
+      - uses: actions/checkout@v2
+      - 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.repository_owner }}
+          password: ${{ secrets.PAT }}
+
+      - name: Push Docker image
+        uses: docker/build-push-action@v2
+        with:
+          file: ./Dockerfile
+          push: true
+          tags: |
+            ghcr.io/${{ github.repository }}:main