[release-1.x backport] logName(): lazily lookup userName instead of on init()

[thaJeztah]

backport of #143 for the release-1.x branch

What this PR does / why we need it:

Commit c46b9e1 (#123, and backported to v1 through #124) implemented a workaround for situations on Windows where user.Current() was not available.

On Linux/Unix ennvironments, user.Current() may be calling (among others) getgrnam_r (https://linux.die.net/man/3/getgrgid_r), which:

Returns a pointer to a structure containing the broken-out fields of
the record in the group database (e.g., the local group file /etc/group,
NIS, and LDAP) that matches the group name name.

This means that the init() function might be making network connections,
which is not desirable.

This patch changes the lookup to be performed lazily. A sync.Once was
added so that lookup is only performed once (to keep the behavior that
was previously provided by using init().

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):

Relates to #123
Relates to docker/cli#2420

Special notes for your reviewer:

Please confirm that if this PR changes any image versions, then that's the sole change this PR makes.

If accepted, I would like this to see this backported to the v1 branch (and possibly included in a v1.0.1 or v1.0.2 release)

Release note:

Perform userName lookup lazily, instead of on init()

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: thaJeztah
To complete the pull request process, please assign lavalamp
You can assign the PR to them by writing /assign @lavalamp in a comment when ready.

The full list of commands accepted by this bot can be found here.

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[thaJeztah]

/assign @neolit123
/assign @dims

[neolit123]

ideally backports should include only critical bug fixes. how can we tell if is this one?

[thaJeztah]

Perhaps it's not critical (let me /cc @tiborvass on this, who I think looked into this at some point w.r.t. what kind of calls user.Current() made)

[dims]

/close

let's close this for now.

[k8s-ci-robot]

@dims: Closed this PR.

In response to this:

/close

let's close this for now.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.