Bot build: add workflow to publish docker container of headless-game-runner (bot)

.github/workflows/build-headless-docker.yml

@@ -0,0 +1,33 @@
+# Runs when master branch is updated.
+# Creates & pushes a docker container for bots.
+
+name: Master Branch Build - Build Bot Docker Container
+on:
+  push:
+    branches:
+      - master
+jobs:
+  build:
+    env:
+      IS_MASTER_BUILD: ${{ github.event_name == 'push' && github.ref == 'refs/heads/master' }}
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Set up JDK 11
+        uses: actions/setup-java@v4
+        with:
+          distribution: temurin
+          java-version: 11
+      - name: Login to GitHub Container Registry
+        if: ${{ env.IS_MASTER_BUILD }}
+        uses: docker/login-action@v1
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+      - name: build headless docker container
+        if: ${{ env.IS_MASTER_BUILD }}
+        run: |
+          ./gradlew :game-app:game-headless:shadowJar
+          docker build game-app/game-headless --tag ghcr.io/triplea-game/bot:latest
+          docker push ghcr.io/triplea-game/bot:latest