added docker image support

* added environment variable support
* added docker instructions
* added .env hint

docker-compose.yaml

@@ -0,0 +1,22 @@
+version: "3.9"
+services:
+  init:
+    image: alpine:latest
+    user: "root"
+    group_add:
+      - '${GROUP_ID}'
+    volumes:
+      - ${OUTDIR}:/backup
+    command: chown -R ${USER_ID}:${USER_ID} /backup
+
+  kleingebaeck:
+    container_name: kleingebaeck
+    user: "${USER_ID}:${USER_ID}"
+    volumes:
+      - ${OUTDIR}:/backup
+    working_dir: /backup
+    build: .
+    image: kleingebaeck:latest
+    depends_on:
+      init:
+        condition: service_completed_successfully