dev_container: Parse env vars and docker labels with = in values correctly

[yeetypete]

Fixes a parsing issue where docker env var key/value pairs can contain an "=" character in the value. This is pretty common and present in all nvidia/cuda docker images. Also adds some tests for env var parsing.

Self-Review Checklist:

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments
• The content is consistent with the UI/UX checklist
• Tests cover the new/changed behavior
• Performance impact has been considered and is acceptable

Release Notes:

• Fixed a parsing failure where docker env var key/value pairs can contain an "=" character in the value.

[Vastargazing]

There's a similar bug still present in deserialize_labels > visit_seq in the same file (the array-of-strings branch). It still uses the old split("=").collect() + parts.len() != 2 pattern. Any label whose value contains = will be silently dropped no error, no log.

The fix is a one-liner using the same approach you already introduced:

.filter_map(|v| {
    let (key, value) = v.split_once('=')?;
    Some((key.to_string(), value.to_string()))
})

it would also be good to add a test for it, mirroring should_parse_env_var_with_equals_in_value

[KyleBarton]

Good catch, thanks for the contribution!

[KyleBarton]

@zed-zippy approved