Support building OSI images outside of container

[rezib]

For building some OSI output formats (eg. gpt_ext4 or gpt_btrfs), mkosi needs to allocate loop device with losetup. Unfortunately, this cannot work in containers due to missing virtualization support of loop devices in Linux. This fails with the following error:

$ fatbuildrctl --uri dbus://system/hpckit build -a node-debian11 -s firehpc/envs --watch
Submitted node-debian11 build task 831c1f39-eb1e-4aae-8ab1-61962ccb5bac
 ⚬ INFO: Running build 831c1f39-eb1e-4aae-8ab1-61962ccb5bac
 ⚬ INFO: Extracting tarball /run/fatbuildr/fatbuildr-artifact-b9aedccu.tar.xz in destination /var/lib/fatbuildr/workspaces/hpckit/831c1f39-eb1e-4aae-8ab1-61962ccb5bac
 ⚬ INFO: Building the OS image based node-debian11
‣ Changing ownership of output file /var/lib/fatbuildr/workspaces/hpckit/831c1f39-eb1e-4aae-8ab1-61962ccb5bac/debian~bullseye to user fatbuildr…
‣ Changed ownership of /var/lib/fatbuildr/workspaces/hpckit/831c1f39-eb1e-4aae-8ab1-61962ccb5bac/debian~bullseye
‣ Removing output files…
‣ Detaching namespace
‣ Setting up temporary workspace.
‣ Temporary workspace set up in /var/tmp/mkosi-hf5j_7l3
‣ Running second (final) stage…
‣  Creating image with partition table…
Disk /var/lib/fatbuildr/workspaces/hpckit/831c1f39-eb1e-4aae-8ab1-61962ccb5bac/debian~bullseye/.mkosi-9jfpbe27: 3 GiB, 3221266432 bytes, 6291536 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes

>>> Script header accepted.
>>> Script header accepted.
>>> Script header accepted.
>>> Created a new GPT disklabel (GUID: CC8A1A7E-B84A-A640-A914-8F4F2E3ABBA5).
/var/lib/fatbuildr/workspaces/hpckit/831c1f39-eb1e-4aae-8ab1-61962ccb5bac/debian~bullseye/.mkosi-9jfpbe27p1: Created a new partition 1 of type 'Linux root (x86-64)' and of size 3 GiB.
/var/lib/fatbuildr/workspaces/hpckit/831c1f39-eb1e-4aae-8ab1-61962ccb5bac/debian~bullseye/.mkosi-9jfpbe27p2: Done.

New situation:
Disklabel type: gpt
Disk identifier: CC8A1A7E-B84A-A640-A914-8F4F2E3ABBA5

Device                                                                                                      Start     End Sectors Size Type
/var/lib/fatbuildr/workspaces/hpckit/831c1f39-eb1e-4aae-8ab1-61962ccb5bac/debian~bullseye/.mkosi-9jfpbe27p1    40 6291495 6291456   3G Linu

The partition table has been altered.
‣  Created image with partition table as /var/lib/fatbuildr/workspaces/hpckit/831c1f39-eb1e-4aae-8ab1-61962ccb5bac/debian~bullseye/.mkosi-9jfpbe27
‣  Attaching /var/lib/fatbuildr/workspaces/hpckit/831c1f39-eb1e-4aae-8ab1-61962ccb5bac/debian~bullseye/.mkosi-9jfpbe27 as loopback…
losetup: cannot find an unused loop device
Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "/usr/lib/python3.11/site-packages/mkosi/__main__.py", line 45, in <module>
    main()
  File "/usr/lib64/python3.11/contextlib.py", line 81, in inner
    return func(*args, **kwds)
           ^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/site-packages/mkosi/__main__.py", line 41, in main
    run_verb(a)
  File "/usr/lib/python3.11/site-packages/mkosi/__init__.py", line 8254, in run_verb
    manifest = build_stuff(config)
               ^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/site-packages/mkosi/__init__.py", line 7559, in build_stuff
    image = build_image(state, manifest=manifest)
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/site-packages/mkosi/__init__.py", line 7236, in build_image
    with attach_base_image(state.config.base_image, state.partition_table) as base_image, \
  File "/usr/lib64/python3.11/contextlib.py", line 137, in __enter__
    return next(self.gen)
           ^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/site-packages/mkosi/__init__.py", line 982, in attach_image_loopback
    with get_loopdev(image) as loopdev, flock(loopdev):
  File "/usr/lib64/python3.11/contextlib.py", line 137, in __enter__
    return next(self.gen)
           ^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/site-packages/mkosi/__init__.py", line 963, in get_loopdev
    c = run(["losetup", "--find", "--show", "--partscan", f.name], stdout=subprocess.PIPE, text=True)
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/site-packages/mkosi/backend.py", line 860, in run
    return subprocess.run(cmdline, check=check, stdout=stdout, stderr=stderr, env={**os.environ, **env}, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib64/python3.11/subprocess.py", line 571, in run
    raise CalledProcessError(retcode, process.args,
subprocess.CalledProcessError: Command '['losetup', '--find', '--show', '--partscan', '/var/lib/fatbuildr/workspaces/hpckit/831c1f39-eb1e-4aae-8ab1-61962ccb5bac/debian~bullseye/.mkosi-9jfpbe27']' returned non-zero exit status 1.
 ⚬ ERROR: error while running task 831c1f39-eb1e-4aae-8ab1-61962ccb5bac: Command /usr/libexec/fatbuildr/u-nspawn --directory /var/lib/fatbuildr/images/hpckit/osi.img --bind /usr/share/fatbuildr/images/common --quiet --register=no --keep-unit --bind /var/lib/fatbuildr/workspaces/hpckit/831c1f39-eb1e-4aae-8ab1-61962ccb5bac --bind /var/cache/fatbuildr/hpckit/node-debian11 --bind /var/lib/fatbuildr/registry/hpckit/osi --setenv SUDO_UID=998 /usr/bin/mkosi --nspawn-keep-unit --default /var/lib/fatbuildr/workspaces/hpckit/831c1f39-eb1e-4aae-8ab1-61962ccb5bac/osi/node-debian11.mkosi --output-dir /var/lib/fatbuildr/workspaces/hpckit/831c1f39-eb1e-4aae-8ab1-61962ccb5bac --image-id node-debian11 --image-version 1 --checksum failed with exit code 1
 ⚬ INFO: Task failed

The solution would be to support building OSI images outside containers, by running mkosi directly on the host system.