Fix container log following when the log is empty

[i098]

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Motivation and Context

container logs -f exits when its initial read finds an empty log.
Skip the empty initial read without returning, so follow mode can wait for new output.
Keep the full-file read to preserve existing output before following.
Add a regression test for output appended to an initially empty log.

Leave container machine logs unchanged because it has a separate implementation and this issue concerns container logs.

Closes #2374.

Testing

• Tested locally
• Added/updated tests
• Added/updated docs

Repro command, with an initially empty container log:

container logs -f logempty

Verified at the file-tail level on macOS: before the fix, it returns immediately; after, it waits and prints hello when appended.
ContainerLogsTests.followsInitiallyEmptyLog fails before the fix and passes after it.

I am new to open source, and this pull request was made with AI assistance.