Configure Docker build workflow with environment variables

Added environment variables for Docker registry and image name.

Author: shuesken

.github/workflows/docker-build.yml

@@ -1,9 +1,20 @@
 name: build
 on:
   workflow_dispatch:
+env:
+  # Use docker.io for Docker Hub if empty
+  REGISTRY: ghcr.io
+  # github.repository as <account>/<repo>
+  IMAGE_NAME: ${{ github.repository }}
+
+
 jobs:
   build:
+
     runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      packages: write
     steps:
       - name: Checkout repository
         uses: actions/checkout@v2