Docker: support PidMode for setting PID namespace
What does this MR do?
Add support for specifying the PID mode in Docker executor.
Why was this MR needed?
Addresses #36847
What's the best way to test this MR?
Add a pid_mode setting to the Docker executor configuration and check that the container runs with the appropriate flag.
What are the relevant issue numbers?
Other considerations
I'm not sure whether the option should also apply to service containers, and whether the new parameter should be documented in docs/configuration/advanced-configuration.md. Decided to start with a minimal change in line with !3781 (merged) as a template: let me know if it would be tweaked, or feel free to adjust the MR accordingly.