docker: bower and user improvements
docker: bower and user improvements

* Amends installation instructions to install all bower prerequisites.
  Shows example how to run bower, should generated components not be
  committed in the sources.

* Sets container user to `www-data` and forces its UID to be 1000, which
  should match host user identity in most scenarios.

* Launches explicit `apt-get clean` to reduce the size of the container.

Signed-off-by: Tibor Simko <c7116e45300b163846dc01f3ed02116e3f8ef7a5@cern.ch>

SamiHiltunen/invenio-software.org,tiborsimko/inveniosoftware.org,SamiHiltunen/invenio-software.org,SamiHiltunen/invenio-software.org,SamiHiltunen/invenio-software.org