Elastic Agent docker user should be numeric

- Relates https://github.com/elastic/beats/pull/41197

Make the same change that was done for Beats in https://github.com/elastic/beats/pull/41197 for Elastic Agent, for example the user below should be numeric instead.

https://github.com/elastic/elastic-agent/blob/f32807c8af3abb08d211948bfe63e04af44d8ef3/dev-tools/packaging/packages.yml#L521

This makes the container with more restricted security contexts using `runAsNonRoot` set to `true` which requires a numeric user to verify the user is not the root user.

## Acceptance Criteria

Add a test that proves the agent containers can run as unprivileged in security contexts that set `runAsNonRoot` to `true`

Start with dev-tools/packaging/packages.yml at the referenced user entry and compare the equivalent change in Beats PR 41197. Add the Elastic Agent container test requested in the acceptance criteria, then verify that the container runs in a security context with runAsNonRoot set to true and that the configured user is numeric.

Written by the indexing model from the issue text.