Build Dockerfile.ci using Github workflows (#325)

We have to build and push the docker image used in CI manually. This PR builds that image automatically and pushes it to Github docker repository.

Will start using that image in a follow PR

.github/workflows/publish-ci-docker-image.yml

@@ -0,0 +1,20 @@
+name: publish-ci-docker-image
+on:
+  push:
+    branches: [ main ]
+jobs:
+  publish-ci-docker-image:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - name: Login to GitHub Container Registry
+        uses: docker/login-action@v1
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+      - name: Build CI Docker image
+        run: |
+          docker build . -f Dockerfile.ci --tag ghcr.io/levkk/pgcat-ci:latest
+          docker run ghcr.io/levkk/pgcat-ci:latest
+          docker push ghcr.io/levkk/pgcat-ci:latest