Add Alpine Linux CI build job (!104)

There have been a number of GParted build issues [1][2] recently on
Alpine Linux because it uses musl libc [3] which is stricter to POSIX,
rather than the GNU C Library (glibc) which has numerous enhancements.
Glibc is used by most Linux distributions, including CentOS and Ubuntu
already used in the GNOME Continuous Integration jobs.  So add a GParted
build job on Alpine Linux to catch these issues in future.  Uses the
docker image of the latest Alpine Linux release.

[1] 3d4b1c1e7b
    Fix NULL == 0 assumption in call to ped_partition_flag_next() (!100)
[2] 45c00927b7
    Use POSIX basename() in BCache_Info.cc (!99)
[3] musl libc
    https://musl.libc.org/

Closes !104 - Add Alpine Linux CI jobs and resolve label and UUID issues
              with FAT16/32

.gitlab-ci.yml

@@ -2,6 +2,16 @@ stages:
   - build
   - test
 
+.alpine_image_template: &alpine_image_definition
+  # Use official Alpine image https://hub.docker.com/_/alpine/.
+  image: alpine:latest
+  before_script:
+    - cat /proc/version
+    - cat /etc/os-release
+    - apk update
+    - apk add gnome-common yelp-tools automake autoconf glib-dev intltool
+              libtool g++ parted-dev gtkmm3-dev itstool make
+
 .centos_image_template: &centos_image_definition
   # Use official CentOS image https://hub.docker.com/_/centos/.
   image: centos:centos7
@@ -103,7 +113,11 @@ stages:
       - ./
     expire_in: 1 week
 
-# Check GParted can be built and installed on CentOS and Ubuntu.
+# Check GParted can be built and installed on Alpine Linux, CentOS and Ubuntu.
+alpine_build:
+  <<: *alpine_image_definition
+  <<: *build_stage_definition
+
 centos_build:
   <<: *centos_image_definition
   <<: *build_stage_definition