Update to use LABEL instead of MAINTAINER

https://docs.docker.com/engine/reference/builder/#maintainer-deprecated

> The MAINTAINER instruction sets the Author field of the generated images. The LABEL instruction is a much more flexible version of this and you should use it instead, as it enables setting any metadata you require, and can be viewed easily, for example with docker inspect. To set a label corresponding to the MAINTAINER field you could use:
>
> ```
> LABEL maintainer="SvenDowideit@home.org.au"
> ```
>
> This will then be visible from docker inspect with the other labels.