feat(ci): better rust CI

.github/workflows/rust.yml

@@ -0,0 +1,56 @@
+# todo: use https://github.com/jpribyl/action-docker-layer-caching
+# todo: make this generic for all Rust images in this repo
+# todo: don't use docker/build-push-action
+# todo: run builds on pull request
+
+name: Build and push API Docker image
+on:
+  push:
+    branches:
+      - main
+      - 'rust-api'
+    paths:
+    - 'lib/pklib/**'
+    - 'services/api/**'
+
+jobs:
+  deploy:
+    runs-on: ubuntu-latest
+    permissions:
+      packages: write
+    if: github.repository == 'PluralKit/PluralKit'
+    steps:
+      - uses: docker/login-action@v1
+        if: ${{ !env.ACT }}
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.CR_PAT }}
+
+      - uses: actions/checkout@v2
+      - uses: docker/setup-buildx-action@v1
+
+      # main docker build
+      - run: echo "BRANCH_NAME=${GITHUB_REF#refs/heads/}" >> $GITHUB_ENV
+      - uses: docker/build-push-action@v2
+        with:
+          # https://github.com/docker/build-push-action/issues/378
+          context: .
+          file: Dockerfile.rust
+          push: false 
+          cache-from: type=gha
+          cache-to: type=gha
+          outputs: .docker-bin
+
+      # add more binaries here
+      - run: |
+          for binary in "api"; do
+            for tag in latest ${{ env.BRANCH_NAME }} ${{ github.sha }}; do
+              cat Dockerfile.bin | sed "s/__BINARY__/$binary/g" | docker build -t ghcr.io/pluralkit/$binary/$tag -f - .
+            done
+            if [ "${{ github.repository }}" == "PluralKit/PluralKit" ]; then
+              docker push ghcr.io/pluralkit/$binary:${{ env.BRANCH_NAME }}
+              docker push ghcr.io/pluralkit/$binary:${{ github.sha }}
+              [ "${{ env.BRANCH_NAME }}" == "main" ] && docker push ghcr.io/pluralkit/$binary:latest
+            fi
+          done