docker: Parameterize loading the loop kernel module

[thetredev]

These changes introduce a config variable LOAD_LOOP_MODULE which can be set to yes (default) or no within the /etc/docker/sv/conf file.

Testing the changes

• I tested the changes in this PR: YES (xlint docker + ./xbps-src pkg -Q docker)
• I tested that the Docker service is working when the loop kernel module is not required (e.g. under WSL): YES (see below)

PS C:\Users\Cheeba> wsl -u root bash -ic "cat /etc/sv/docker/conf"
cat: /etc/sv/docker/conf: No such file or directory
PS C:\Users\Cheeba> wsl -u root bash -ic "sv status docker"
down: docker: 1s, normally up, want up; run: log: (pid 170) 3s
PS C:\Users\Cheeba> wsl -u root bash -ic "sv restart docker"
timeout: down: docker: 1s, normally up, want up
PS C:\Users\Cheeba> wsl -u root bash -ic "vim /etc/sv/docker/conf"
PS C:\Users\Cheeba> wsl -u root bash -ic "cat /etc/sv/docker/conf"
LOAD_LOOP_MODULE=yes
PS C:\Users\Cheeba> wsl -u root bash -ic "sv restart docker"
timeout: down: docker: 1s, normally up, want up
PS C:\Users\Cheeba> wsl -u root bash -ic "sv status docker"
down: docker: 0s, normally up, want up; run: log: (pid 170) 35s
PS C:\Users\Cheeba> wsl -u root bash -ic "vim /etc/sv/docker/conf"
PS C:\Users\Cheeba> wsl -u root bash -ic "cat /etc/sv/docker/conf"
LOAD_LOOP_MODULE=no
PS C:\Users\Cheeba> wsl -u root bash -ic "sv restart docker"
ok: run: docker: (pid 479) 1s
PS C:\Users\Cheeba> wsl -u root bash -ic "sv status docker"
run: docker: (pid 479) 2s; run: log: (pid 170) 45s
PS C:\Users\Cheeba> wsl -u root bash -ic "docker ps"
CONTAINER ID   IMAGE     COMMAND   CREATED   STATUS    PORTS     NAMES
PS C:\Users\Cheeba> wsl -u root bash -ic "docker version"
Client:
 Version:           24.0.6
 API version:       1.43
 Go version:        go1.21.0
 Git commit:        tag v24.0.6
 Built:             Wed Sep  6 13:42:27 2023
 OS/Arch:           linux/amd64
 Context:           default

Server:
 Engine:
  Version:          24.0.6
  API version:      1.43 (minimum version 1.12)
  Go version:       go1.21.0
  Git commit:       tag v24.0.6
  Built:            Wed Sep  6 13:42:27 2023
  OS/Arch:          linux/amd64
  Experimental:     false
 containerd:
  Version:          1.7.7
  GitCommit:        UNSET
 runc:
  Version:          1.1.9
  GitCommit:
 docker-init:
  Version:          0.19.0
  GitCommit:
PS C:\Users\Cheeba>

Motivation behind the changes

I played around with using Void Linux under WSL2 and struggled to get the Docker service started up. Eventually I realized that the modprobe -q loop command would always fail under WSL2, because there is no directory /lib/modules, so modprobe is essentially useless under WSL2. I think there are other mechanisms involved when loading the WSL2 kernel and modules. Either way, Docker runs very well with these changes.

I understand that Void Linux is not officially supported by Microsoft (and probably vice-versa), and there's manual steps involved installing it, but I thought why not make Docker run for everybody nonetheless?

These changes should make the Docker service run both when the loop module is available and has to be loaded (default), as well as in systems where it's not available or not required, like WSL2.