fix hadolint DL3009 warning for cleaning up the apt-get lists after installation

[krasingneves]

Standard procedure for creating Docker containers where I work includes running the Dockerfiles through hadolint. With the current reference Dockerfile, hadolint gives the reference architecture an error for violating DL3009, which is a recommendation aimed at reducing the container size.

As we know, the Matlab container can be quite large, so any size reduction is generally favorable.

[krasingneves]

@josmartin - forgot to mention this but figured it was simple enough, you might take the pull request :)