remove deprecated maintainer from dockerfile

.github/workflows/ci.yaml

@@ -21,6 +21,14 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v1    
       - name: lint zammad dockerfile
+        uses: brpaz/hadolint-action@master
+        with:
+          dockerfile: containers/zammad-elasticsearch/Dockerfile
+      - name: lint elasticsearch dockerfile
+        uses: brpaz/hadolint-action@master
+        with:
+          dockerfile: containers/zammad-postgresql/Dockerfile
+      - name: lint postgresql dockerfile
         uses: brpaz/hadolint-action@master
         with:
           dockerfile: containers/zammad/Dockerfile                    

containers/zammad-elasticsearch/Dockerfile

@@ -1,5 +1,4 @@
 FROM docker.elastic.co/elasticsearch/elasticsearch:7.6.0
-MAINTAINER Zammad <info@zammad.org>
 ARG BUILD_DATE
 
 LABEL org.label-schema.build-date="$BUILD_DATE" \

containers/zammad-postgresql/Dockerfile

@@ -1,6 +1,5 @@
 FROM postgres:9.6.17-alpine
 
-MAINTAINER Zammad <info@zammad.org>
 ARG BUILD_DATE
 
 LABEL org.label-schema.build-date="$BUILD_DATE" \

containers/zammad/Dockerfile

@@ -2,7 +2,6 @@ FROM ruby:2.5.5-slim AS builder
 # note: zammad is currently incompatible to alpine because of: 
 # https://github.com/docker-library/ruby/issues/113
 
-MAINTAINER Zammad <info@zammad.org>
 ARG BUILD_DATE
 ARG DEBIAN_FRONTEND=noninteractive