Add CI jobs for building and testing packages on Debian 9

changed milestone to %9.4

@marin The tests depend on the smaller image being available. I've tried them on my fork and they ran successfully https://gitlab.com/balasankarc/omnibus-gitlab/pipelines/8966833. Once the smaller image is available, we can just retry the failed builds and they will run fine.

unmarked as a Work In Progress

assigned to @marin

approved this merge request

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approved this merge request

mentioned in commit 59764f6d

merged

changed milestone to %9.3

mentioned in commit f4f713c9

mentioned in merge request !1648 (merged)

Conflicts found when picking this MR. Created MR !1648 (merged) to resolve it

!1648 (merged) was merged into 9.3 RC3.

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