Guest hostname handling improvements (with override)

[comunidadio]

This PR improves guest hostname handling by:

• defaulting guest hostname to vm's id instead of "matchlock" for all
• setting hostname via kernel arg so it's set before userspace and guest-init
• having guest-init also write /etc/hostname to improve compatibility with tools that expect /etc/hostname to match kernel's hostname
• allowing hostname override via Config or MATCHLOCK_HOSTNAME

Without override:

➜  matchlock git:(feat/id-hostname) ✗ ./bin/matchlock run --image alpine:latest --rm -it sh
~ # hostname
vm-95c743d6
~ # cat /etc/hostname
vm-95c743d6
~ # exit

With override:

➜  matchlock git:(feat/id-hostname) ✗ MATCHLOCK_HOSTNAME=something.internal ./bin/matchlock run --image alpine:latest --rm -it sh
~ # hostname
something.internal
~ # cat /etc/hostname
something.internal
~ # exit

[jingkaihe]

Thanks for contiributing. Can we do --name or --hostname in run command, as the env var sometimes is not the most obvious thing?
Also /etc/hosts injection will also be nice (it was on my list but didn't get the chance to do it)

[comunidadio]

Makes sense, removed MATCHLOCK_HOSTNAME override, exposing --hostname instead.

Good idea re: /etc/hosts!

For /etc/hosts injection are you comfortable adding a dependency such as https://github.com/txn2/txeh ?
Seems to me it would be the simplest and safest way to precisely inject our guest hostname on localhost lines regardless of how an OCI image organizes its /etc/hosts, without having to handle all the intricacies of the format ourselves.
And that may be better done in another PR probably.

[jingkaihe]

I'd prefer avoiding dependencies and just write lines into the /etc/hosts, at least to begin with.

Typically OCI images do not have /etc/hosts as they are typically mounted, therefore I believe it is fairly safe to override it manually.

[comunidadio]

Fair enough, if we're not adding a dependency I'll just roll it into this PR shortly.

[comunidadio]

Done!

➜  matchlock git:(feat/id-hostname) ✗ ./bin/matchlock run --image alpine:latest --hostname override.internal --rm -it sh
~ # cat /etc/hosts
127.0.0.1       localhost localhost.localdomain override.internal
::1             localhost localhost.localdomain override.internal
~ # ping override.internal
PING override.internal (::1): 56 data bytes
64 bytes from ::1: seq=0 ttl=64 time=0.081 ms

[comunidadio]

I'll add support for adding the file from scratch as well on images that do not have /etc/hosts at all (eg. ubuntu:latest)

[comunidadio]

Done!
Interestingly in the case of ubuntu:latest, there is a /etc/hosts file but it's empty.
We handle now all cases I can think of (no file, empty file, standard file, custom-built file)

➜  matchlock git:(feat/id-hostname) ✗ ./bin/matchlock run --image ubuntu:latest --hostname override.internal --rm -it sh
# cat /etc/hosts
127.0.0.1       localhost localhost.localdomain override.internal
::1             localhost localhost.localdomain override.internal
#