Each edit to the h-m-m script was triggering a full image rebuild,
including the slow apt-get and docker-php-ext-install steps.
I figure that the h-m-m script itself is more likely to change than the
system installs.
Moving the COPY to after those operations means that each time h-m-m
changes, the `docker build` can use the cache for the slow operations,
and just add a new layer for the COPY at the end.
Plus `chmod +x h-m-m` is unnecessary as h-m-m already has the executable
bit set.