fix: correct creation of container /etc/hostname file

[dagraff-cisco]

Does this PR introduce a user-facing change?

None

As noted in #22729:

As per debian's hostname (5) man page:
> The file should contain a single newline-terminated hostname string. Comments (lines starting with a "#") are ignored. The hostname should be composed of up to 64 7-bit ASCII lower-case alphanumeric characters or hyphens forming a valid DNS domain name. It is recommended that this name contains only a single label, i.e. without any dots. Invalid characters will be filtered out in an attempt to make the name valid, but obviously it is recommended to use a valid name and not rely on this filtering.

I have made the change noted in the issue and provided a unit test to ensure that the value written is correct.

[openshift-ci]

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[Luap99]

Thanks for the PR, given #22859 was opened first and already received comments I prefer to continue there and close this one