Set up publishing to GitHub Packages

See:

#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

@@ -42,6 +42,7 @@ subprojects {
   apply plugin: 'signing'
   apply plugin: 'maven'
   apply plugin: 'jacoco'
+  apply plugin: 'maven-publish'
 
   sourceCompatibility = 1.8
   targetCompatibility = 1.8
@@ -197,9 +198,43 @@ gradle.projectsEvaluated {
       }
     }
 
+    //'upload' vs. 'publish, see 
+    //https://medium.com/dot-debug/deploying-artifacts-to-maven-using-gradle-b669acc1b6f8'
+    /*
     task publish {
       dependsOn tasks.uploadArchives
     }
+    */
+
+    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-core"
+          credentials {
+            username = System.getenv("GITHUB_ACTOR")
+            password = System.getenv("GITHUB_TOKEN")
+          }
+        }
+      }
+    }
   }
 }