set virtual grain for docker using systemd and virt-what

[furlongm]

What does this PR do?

Correctly handle the systemd-detect-virt and virt-what output to identify a
Docker container running systemd as what it is instead of as a physical machine.

What issues does this PR fix or reference?

Fixes #67905

Previous Behavior

# systemd-detect-virt
docker

# virt-what
docker

# salt-call --local grains.get virtual
local:
    physical

New Behavior

# systemd-detect-virt
docker

# virt-what
docker

# salt-call --local grains.get virtual
local:
    container

Merge requirements satisfied?

[NOTICE] Bug fixes or features added to Salt require tests.

• Docs
• Changelog - https://docs.saltproject.io/en/master/topics/development/changelog.html
• Tests written/updated

Commits signed with GPG?

Yes

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[furlongm]

@dmurphy18 this is similar to the podman issue #67733, lmk if anything else is required, thanks!

[furlongm]

@dwoz would it be possible to get a review on this PR? Thanks.

[welcome]

Congratulations on your first PR being merged! 🎉