Add support for cgroup_parent setting on Docker executor

What does this MR do?

Adds support for the cgroup_parent option Docker executor.

Why was this MR needed?

Without this MR there are some scenarios where setting CPU/memory limits for Docker executor cannot work (e.g. rootless environment without Systemd)

What's the best way to test this MR?

Shorter test

• Run a CI job
• Execute docker inspect <my job's container id> | grep "CgroupParent" on runner's host
• Result should be "CgroupParent": "",
• Now add cgroup_parent=<name of the new cgroup> to runner's config.toml
• Run a CI job
• Execute docker inspect <my job's container id> | grep "CgroupParent" on runner's host
• Result should be "CgroupParent": "<name of the new cgroup>",

Longer test

• Prepare a rootless Podman environment (at least Podman 4.x) on Alpine (with OpenRC)
• Configure runner to use the rootless Podman socket
• Add cpuset_cpus=0 to runner's config.toml
• Run a CI job that executes nproc
• This will either fail or nproc will report a value higher than 1
• Now create a cgroup and assign it to the user that owns the socket
• Add cgroup_parent=<name of the new cgroup> to runner's config.toml
• Re-run the previous job
• nproc will now print 1

What are the relevant issue numbers?

#28644