build: Add action to build and automatically release files (#170)

Auto-release is gonna get a nice live test at a later date.

.github/workflows/ci.yml

@@ -0,0 +1,93 @@
+name: OpenInv CI
+
+on:
+  push:
+  create:
+    types: [tag]
+  pull_request_target:
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: true
+    steps:
+      - name: Checkout Code
+        uses: actions/checkout@v2
+
+      - name: Set Up Java
+        uses: actions/setup-java@v1
+        with:
+          java-version: 1.8
+
+      # Use cache to speed up build
+      - name: Cache Maven Repo
+        uses: actions/cache@v2
+        id: cache
+        with:
+          path: ~/.m2/repository
+          key: ${{ runner.os }}-maven-${{ hashFiles('**/pom.xml') }}
+
+      # If the cache was not present, run BuildTools to install the relevant versions to Maven.
+      # This will take approximately forever.
+      - name: Install Spigot Dependencies
+        if: steps.cache.outputs.cache-hit != 'true'
+        run: |
+          mkdir ~/buildtools
+          cd ~/buildtools
+          wget https://hub.spigotmc.org/jenkins/job/BuildTools/lastSuccessfulBuild/artifact/target/BuildTools.jar
+          java -jar BuildTools.jar --rev 1.8.8
+          java -jar BuildTools.jar --rev 1.15.2
+          java -jar BuildTools.jar --rev 1.16.3
+          java -jar BuildTools.jar --rev 1.16.4
+
+      - name: Build With Maven
+        run: mvn -e clean package -am -P all
+
+      # Upload artifacts
+      - name: Upload Distributable Jar
+        id: upload-final
+        uses: actions/upload-artifact@v2
+        with:
+          name: dist
+          path: ./target/OpenInv.jar
+      - name: Upload API Jar
+        id: upload-api
+        uses: actions/upload-artifact@v2
+        with:
+          name: api
+          path: ./api/target/openinvapi*.jar
+
+  release:
+    name: Create Github Release
+    needs: [ build ]
+    if: github.event_name == 'create' && github.event.ref_type == 'tag'
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout Code
+        uses: actions/checkout@v2
+
+      - name: Download Artifacts
+        uses: actions/download-artifact@v2
+
+      - name: Create Release
+        id: create-release
+        uses: actions/create-release@v1
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        with:
+          tag_name: ${{ github.ref }}
+          release_name: Release ${{ github.ref }}
+          draft: true
+          prerelease: false
+
+      - name: Upload Release Asset
+        id: upload-release-asset
+        uses: actions/upload-release-asset@v1.0.2
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        with:
+          upload_url: ${{ steps.create-release.outputs.upload_url }}
+          asset_path: ./OpenInv.jar
+          asset_name: OpenInv.jar
+          asset_content_type: application/java-archive