Set up publishing to GitHub Packages

See:

#356
https://gitlab.com/oersi/oersi-etl/-/issues/59

.github/workflows/publish.yml

@@ -0,0 +1,17 @@
+name: Publish package to GitHub Packages
+on:
+  release:
+    types: [created]
+jobs:
+  publish:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions/setup-java@v1
+        with:
+          java-version: 1.8
+      - name: Publish package
+        run: gradle publish
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+

build.gradle

@@ -338,3 +338,16 @@ def static extractVersionFromBranch(branch) {
   }
   return matcher.group(1)
 }
+
+publishing {
+  repositories {
+    maven {
+      name = "GitHubPackages"
+      url = "https://maven.pkg.github.com/metafacture/metafacture-core"
+      credentials {
+        username = System.getenv("GITHUB_ACTOR")
+        password = System.getenv("GITHUB_TOKEN")
+      }
+    }
+  }
+}