feat(ci): use ghcr and auto release

- Pushes images to GitHub Container Registry (GHCR) to deprecate Docker Hub
- Auto releases for nightly
- Auto releases for new release

.github/workflows/nightly-release.yaml

@@ -0,0 +1,27 @@
+name: Nightly Release
+on:
+  schedule:
+    - cron: '0 0 * * *'
+  workflow_dispatch: {}
+jobs:
+  build-release:
+    name: Build and release Docker image
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v2
+      - name: Update version.go
+        run: |
+          _version=v$(cat version.txt)
+          sed -Ei "s/VERSION = \"v0.0.0\"/VERSION = \"${_version}-nightly\"/g" version/version.go
+      - name: Login into GitHub Container Registry
+        run: echo ${{ secrets.CR_PAT }} | docker login ghcr.io -u $GITHUB_ACTOR --password-stdin
+      - name: Build Docker image
+        run: |
+          docker build \
+          -t "ghcr.io/${GITHUB_REPOSITORY}:${GITHUB_SHA:0:7}" \
+          -t "ghcr.io/${GITHUB_REPOSITORY}:nightly" .
+      - name: Release Docker image
+        run: |
+          docker push "ghcr.io/${GITHUB_REPOSITORY}:${GITHUB_SHA:0:7}"
+          docker push "ghcr.io/${GITHUB_REPOSITORY}:nightly"