Introducing shellcheck in gitlab CI

Quoting all variables.
Changing pkg_list to be an array instead of a string for easier
handling. Using read to properly populate pkg_list from OPTARG with
stripped whitespaces.
Not exporting iso_label anymore as there seems to be no reason to do so.
Introducing line breaks.

Setting max_line_length to 120.
Adding a section for YAML files (e.g. .gitlab-ci.yml).

Quting nearly all variables.
Introducing line breaks.

Quoting all variables.
Introducing line breaks.

Adding gitlab CI for shelleck linting of the config build scripts, mkarchiso and startup scripts in releng.

Closes #19
1 job for master in 27 seconds (queued for 2 seconds)