Set up publishing to GitHub Packages

See:

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

.github/workflows/publish.yml

@@ -0,0 +1,16 @@
+name: Publish package to GitHub Packages
+on:
+  push
+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

@@ -43,6 +43,7 @@ subprojects {
   apply plugin: 'eclipse'
   apply plugin: 'java'
   apply plugin: 'org.xtext.builder'
+  apply plugin: 'maven-publish'
 
   apply from: "${rootDir}/gradle/source-layout.gradle"
 
@@ -73,6 +74,35 @@ subprojects {
 
     options.compilerArgs << '-Werror'
   }
+
+  publishing {
+    publications {
+      mavenArtifacts(MavenPublication) {
+        from components.java
+        afterEvaluate {
+          groupId = project.group
+          artifactId = project.name
+        }
+      }
+    }
+    repositories {
+      maven {
+        name = 'LocalEmbedded'
+        // run generated gradle task `./gradlew 
+        // publishMavenArtifactsPublicationToLocalEmbeddedRepository`
+        // to publish all subprojects into the same local embedded repo:
+        url = "file://${rootDir}/build/repo"
+      }
+      maven {
+        name = "GitHubPackages"
+        url = "https://maven.pkg.github.com/metafacture/metafacture-fix"
+        credentials {
+          username = System.getenv("GITHUB_ACTOR")
+          password = System.getenv("GITHUB_TOKEN")
+        }
+      }
+    }
+  }
 }
 
 task installServer(type: Copy) {