Replace deprecated instruction

MAINTAINER is deprecated, replacing with LABEL as recommended by
https://docs.docker.com/engine/reference/builder/#maintainer-deprecated

Signed-off-by: Charlotte Mach <charlotte.mach@fs.lmu.de>
(cherry picked from commit aa4cb24739a12d82c46eddc8c365c72988fbf8f3)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
Upstream-commit: 01fc5a1ec3349b2ea6b7e1f0a335a7e901ab6cd1
Component: cli

Author: charlottemach

components/cli/docs/reference/commandline/pull.md

@@ -160,7 +160,7 @@ Digest can also be used in the `FROM` of a Dockerfile, for example:
 
 ```dockerfile
 FROM ubuntu@sha256:45b23dee08af5e43a7fea6c4cf9c25ccf269ee113168c19722f87876677c5cb2
-MAINTAINER some maintainer <maintainer@example.com>
+LABEL maintainer="some maintainer <maintainer@example.com>"
 ```
 
 > **Note**

components/cli/man/src/image/pull.md

@@ -111,7 +111,7 @@ pull the above image by digest, run the following command:
 Digest can also be used in the `FROM` of a Dockerfile, for example:
 
     FROM ubuntu@sha256:45b23dee08af5e43a7fea6c4cf9c25ccf269ee113168c19722f87876677c5cb2
-    MAINTAINER some maintainer <maintainer@example.com>
+    LABEL maintainer="some maintainer <maintainer@example.com>"
 
 > **Note**: Using this feature "pins" an image to a specific version in time.
 > Docker will therefore not pull updated versions of an image, which may include