feat(gateway): add Dockerfile and GH Actions build script

.dockerignore

@@ -3,6 +3,7 @@
 
 # Include project code and build files
 !PluralKit.*/
+!gateway/
 !Myriad/
 !PluralKit.sln
 !nuget.config
@@ -13,3 +14,4 @@
 # Re-exclude host build artifact directories
 **/bin
 **/obj
+**/target

.github/workflows/gateway.yml

@@ -0,0 +1,35 @@
+name: Build gateway Docker image
+
+on:
+  push:
+    branches: [main]
+    paths:
+      - 'gateway/'
+      - 'proto/'
+
+jobs:
+  deploy:
+    runs-on: ubuntu-latest
+    permissions:
+      packages: write
+    if: github.repository == 'xSke/PluralKit'
+    steps:
+      - uses: docker/login-action@v1
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.CR_PAT }}
+      - uses: actions/checkout@v2
+      - run: echo "BRANCH_NAME=${GITHUB_REF#refs/heads/}" >> $GITHUB_ENV
+      - uses: docker/build-push-action@v2
+        with:
+          # https://github.com/docker/build-push-action/issues/378
+          context: .
+          file: Dockerfile.gateway
+          push: true
+          tags: |
+            ghcr.io/pluralkit/gateway:${{ env.BRANCH_NAME }}
+            ghcr.io/pluralkit/gateway:${{ github.sha }}
+            ghcr.io/pluralkit/gateway:latest
+          cache-from: type=registry,ref=ghcr.io/pluralkit/gateway:${{ env.BRANCH_NAME }}
+          cache-to: type=inline

Dockerfile.gateway

@@ -0,0 +1,19 @@
+# twilight requires newer rustc than what is in alpine:latest
+FROM alpine:edge AS builder
+
+RUN apk add cargo
+
+# Precache crates.io index
+RUN cargo search >/dev/null
+
+WORKDIR /build
+COPY proto/ /build/proto
+COPY gateway/ /build/gateway
+
+RUN (cd gateway && cargo build --release)
+
+FROM alpine:latest
+
+COPY --from=builder /build/gateway/target/release/pluralkit /opt/gateway
+
+ENTRYPOINT ["/opt/gateway"]