.gitlab-ci.yml:
Add gitlab CI integration to install requirements for and run `make
check`.

Makefile:
Add Makefile with `check` target to lint scripts with shfmt and
shellcheck and YAML files using yamllint.

prepare_archiso_vm.yml:
Add gitlab CI file that allows remote inclusion from other gitlab CI
files and the execution of `scripts/build_in_archiso_vm.sh`.
It implements the basic `.build` target, which can be extended upon.
By default all required packages (e.g. git and qemu-headless) are
installed to clone the repository and execute the script.

scripts/build_in_archiso_vm.sh:
Add script to execute a build script inside an archiso based VM.
The script is based on the latest development in releng
(https://gitlab.archlinux.org/archlinux/releng/-/blob/f3b03c7dfd12defc99d70e322fd0b571bbddc655/template/build-host.sh)
which has initially been taken from arch-boxes
(https://gitlab.archlinux.org/archlinux/arch-boxes/-/blob/5ca90361a7d446b498909a901779cdff3a4018ad/build-host.sh),
refined for the use in archiso
(https://gitlab.archlinux.org/archlinux/archiso/-/blob/960b988ac6dc962c94bb020aac7fe9a85874aa38/.gitlab/ci/build-host.sh).
Add SPDX license identifier for GPL-3.0-or-later.
Fix script to be compliant with shfmt.

LICENSE:
Add LICENSE file (GPL-3-or-later).

README.rst:
Add initial README explaining what the repository is used for and under
what licensing terms it is distributed.