Address #60

See, for example, https://github.com/moby/moby/issues/18017; ARG may
invalidate following `RUN` statements because it implicitly alters the
build context.

Also, use
* apt.local.conf
* ubuntu-16.04.sources.list

Fix #64

- add org.label-schema LABELs for version, build-date, vcs-url

- use latest 8.0 tomcat version (=8.0.53)
- build and install Apache Tomcat Native Library
- use mirror://mirros.ubuntu.com URL schema in apt.sources to speed up
  downloads
- add org.label-schema LABELs for version, build-date, vcs-url
- reduce multiple apt-get update/install invocations

In certain situations, for example, when checking the "Other versions"
dropdown menu a request (possibly after a redirect) is made to
`port:8443`. Of course, this results in an error (because the
clarin-dspace is behind a reverse proxy).
```
OPTIONS https://clarin.eurac.edu:8443/repository/rest/handle/20.500.12124/6/refbox net::ERR_CONNECTION_TIMED_OUT
```

Here, we make sure tomcat will only redirect to `port:443` which is the
port the reverse proxy serves.