ITS#6207 Add GitLab CI

.gitlab-ci.yml

+stages:
+  - build
+
+build:
+  stage: build
+  script:
+    - apt update
+    - apt install -y build-essential pkg-config automake libsasl2-dev heimdal-multidev libssl-dev libltdl-dev groff-base unixodbc-dev libwiredtiger-dev libperl-dev
+    - ./configure --enable-backends=mod --enable-overlays=mod --enable-modules --enable-dynamic --disable-ndb
+    - make depend
+    - make
+    - ulimit -n 4096 # back-monitor takes a while scanning a long connections array
+    - make test
+  artifacts:
+    name: testdir
+    when: on_failure
+    expire_in: '1 week'
+    paths:
+      - tests/testrun/