Use 'LABEL maintainer=' in Dockerfile

* Use 'LABEL maintainer=' in Dockerfile

This fix is a follow up of 13961 to replace `MAINTAINER`
with `LABEL maintainer=` in Dockerfile. The keyword
`MAINTAINER` has long been deprecated and is replaced by `LABEL`,
which is much more flexible and is easily searchable through `docker
inspect`.

This fix replaces remaining `MAINTAINER` with `LABEL`.

Signed-off-by: Charlie Lewis <clewis@iqt.org>

* Additional `MAITAINER` -> `LABEL`

Signed-off-by: Charlie Lewis <clewis@iqt.org>