add an Alpine Linux build

* .gitlab-ci.yml (alpine-gcc): Here.
13 jobs for next in 158 minutes and 54 seconds (queued for 3 seconds)
Status Name Job ID Coverage
  Build
passed alpine-gcc #21978

00:39:38

passed arch-clang #21979

01:00:22

passed arch-gcc-glibcxxdebug #21980

00:38:42

passed debian-gcc-snapshot #21977

01:33:47

passed debian-stable-gcc #21975

00:37:43

passed debian-unstable-gcc-coverage #21976

00:59:04

88.8%
passed debpkg-stable #21983

01:57:29

passed debpkg-unstable #21984

02:36:02

passed mingw-shared #21981

00:31:26

passed mingw-static #21982

00:30:13

passed rpm-pkg #21985

00:43:14

 
  Publish
passed publish-rpm #21986
dput

00:00:10

passed publish-unstable #21987
dput

00:02:40