Artifacts will only be produced if the OUTPUT_ARTIFACTS variable is not 0.

They take up a lot of space and not all project need to retain them. Most just need the metrics.

Fixes #5.

This gets rid of a warning.

    warning: redirecting to https://gitlab.archlinux.org/archlinux/ci-scripts.git/

scripts/build_in_archiso_vm.sh:
8d96297d introduced a regression by
which the filename of the downloaded latest ISO would still contain the
URL path.
Populate the iso variable with the temporary directory plus the remote
filename for the iso instead.

A symlink named archlinux-x86_64.iso that links to the versioned file is guaranteed to exist. Use it instead of parsing HTML with grep.

See https://github.com/pierres/archiso-manager/commit/7f40e1a2edcf44e5124b87409f6da3e974f7d91d and https://github.com/pierres/archiso-manager/commit/9939d7b2ca53d42df3cb9959cf20260f12987345.

Download the matching signature from archlinux.org and use pacman-key to verify it.

The signature must not be downloaded from the mirrors for safety reasons and the use of pacman-key avoids messing with the user's GPG keyring.
See https://wiki.archlinux.org/title/Installation_guide#Verify_signature

geo.mirror.pkgbuild.com is a GeoDNS mirror that points to the sponsored mirrors.

This avoids the usage of Arch's own infrastructure (mirror.pkgbuild.com) for large downloads.
See infrastructure!522 for details.

{.gitlab-ci.yml,scripts/build_in_archiso_vm.sh}:
Install the latest archlinux-keyring before installing anything else.
This is to make sure to not run into outdated keys upon updating, which
is a problem because we can currently not even ensure a keyring that is
valid longer than one month...
archlinux-keyring#4

* issues/2:
  Copy metrics file, instead of moving it

prepare_archiso_vm.yml:
Copy the metrics file from the output directory, instead of moving it,
as this way it is available for e.g. promotion tooling to use it.

Fixes #2

* issues/1:
  Add gitlab CI integration
  Add Makefile to check files
  Add gitlab CI to prepare archiso VM
  Add script to build in archiso vm

.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.