don't pin digests anymore

The benefit of digest pinning isn't that great for this project really
and it comes at the cost of more issues and additional renovate noise,
so just don't anymore.
Adjust renovate config accordingly as well.

resources/chaosknoten/lists/docker_compose/compose.yaml

@@ -1,7 +1,7 @@
 services:
   mailman-core:
     restart: unless-stopped
-    image: docker.io/maxking/mailman-core:0.5@sha256:cb8e412bb18d74480f996da68f46e92473b6103995e71bc5aeba139b255cc3d2 # Use a specific version tag (tag latest is not published)
+    image: docker.io/maxking/mailman-core:0.5 # Use a specific version tag (tag latest is not published)
     container_name: mailman-core
     hostname: mailman-core
     volumes:
@@ -25,7 +25,7 @@ services:
 
   mailman-web:
     restart: unless-stopped
-    image: docker.io/maxking/mailman-web:0.5@sha256:014726db85586fb53541f66f6ce964bf07e939791cfd5ffc796cd6d243696a18 # Use a specific version tag (tag latest is not published)
+    image: docker.io/maxking/mailman-web:0.5 # Use a specific version tag (tag latest is not published)
     container_name: mailman-web
     hostname: mailman-web
     depends_on:
@@ -56,7 +56,7 @@ services:
       - POSTGRES_DB=mailmandb
       - POSTGRES_USER=mailman
       - POSTGRES_PASSWORD=wvQjbMRnwFuxGEPz
-    image: docker.io/library/postgres:12-alpine@sha256:7c8f4870583184ebadf7f17a6513620aac5f365a7938dc6a6911c1d5df2f481a
+    image: docker.io/library/postgres:12-alpine
     volumes:
       - /opt/mailman/database:/var/lib/postgresql/data
     networks: