Stop xmllint scrollkeeper-omf.dtd fetch failure breaking CI tests (#9)

The GitLab Continuous Integration test stage jobs can fail like this:

    $ make check
    Making check in help
    make[1]: Entering directory `/builds/mfleetwo/gparted/help'
    xmllint --noout --xinclude --dtdvalid '' gparted-C.omf
    warning: failed to load external entity ""
    Could not parse DTD
    xmllint --noout --xinclude --dtdvalid '' gparted-cs.omf
    make[1]: *** [check-doc-omf] Error 2
    make[1]: Leaving directory `/builds/mfleetwo/gparted/help'
    make: *** [check-recursive] Error 1
    ERROR: Job failed: exit code 1

It fails when the site reports that
SourceForge is undergoing maintenance or is temporarily unavailable.  I
have seen this occur on 3 separate occasions in the last 4 weeks since I
started experimenting with GitLab CI, which is rather too often.

Xmllint comes from the GNOME 2 gnome-doc-utils.make rules used to build
and validate GNOME 2 documentation.

Fragment of useful Debian bug report 730688:
    --disable-scrollkeeper requieres scrollkeeper installed

    "You can reproduce the problem in mdbtoools version 0.7.1-1 with no
    network, and rarian-compat not installed.

    When the network is available, buildd downloads the DTD for checks.
    When there is no network, gnome-doc-utils fails.

Fix by:
(1) adding the rarian-compat package to the CI Docker images, which
    provides a local copy of the scrollkeeper-omf.dtd file;
(2) adding the xmllint --nonet option to prevent fetching of DTDs

With reference to earlier commit:
    Reduce dependency on scrollkeeper (#743318)
That commit allowed GParted to be installed on GNOME 3 desktops without
requiring rarian-compat package to be installed.  This commit adds
rarian-compat to the CI images so that 'make check' can succeed without
accessing the Internet.  Just the intricate path to continue to build
and test a GNOME 2 application in a world of GNOME 3 desktops with
beginning to be reduced backward compatibility.

Closes #9 - CI test jobs occasionally fail with xmllint not loading
            external entity
4 jobs for 9-ci-test-jobs-occassionally-fail-with-xmllint-not-loading-external-entity-http-scrollkeeper-sourceforge-net-dtds-scrollkeeper-omf-1-0-scrollkeeper-omf-dtd in 12 minutes and 54 seconds (queued for 3 seconds)
Status Job ID Name Coverage
passed #71103


passed #71104


passed #71105


passed #71106