Issue #211: provide basic .gitlab-ci.yml

[MichaelThumes]

This change is for the rel-10_0 branch.
Please propagate this changes upwards to the other release branches.

When merging up, please change the default value for variable OTOBO_BASE in line 16

value: "rel-10_0"

to match the corresponding otobo-docker version. That is, for

rel-10_1 use rel-10_1
rel-11_0 use rel-11_0
rel_11_1 use rel-11_0

in other words set OTOBO_BASE to the release branch version, except for rel-11_1 which also uses the rel-11_0 otobo-docker image.

Please keep this dev branch open for a bit. There might be further tweaking ;-)