Maintainer is deprecated in Dockerfile

[jonasdlindner]

According to the Dockerfile reference, MAINTAINER has been deprecated in favour of using the LABEL instruction: https://docs.docker.com/engine/reference/builder/#maintainer-deprecated.

In Dockerfile Line 2:
MAINTAINER Mark Gituma <mark.gituma@gmail.com>

a possible fix would be:
LABEL maintainer="Mark Gituma <mark.gituma@gmail.com>"