docker-proxy: ensure cgroup parent for docker is valid

[cheimu]

Signed-off-by: Cheimu xerhoneyfc@gmail.com

Ⅰ. Describe what this PR does

For Docker container runtime, and systemd cgroup driver, the cgroup parent only container pod directory. If pass it a relative directory from kubepods.slice, there will be an OCI error. So this PR:

1. make sure the cgroup parent format for Docker is valid
2. make sure the cgroup parent format for hook server is full path, which will decouple the type of container runtime for hook server.

Ⅱ. Does this pull request fix one issue?

Ⅲ. Describe how to verify it

Ⅳ. Special notes for reviews

[cheimu]

/assign @honpey @ZYecho

[codecov]

Codecov Report

Merging #281 (ace03ea) into main (c5ee5fa) will increase coverage by 0.05%.
The diff coverage is 90.90%.

@@            Coverage Diff             @@
##             main     #281      +/-   ##
==========================================
+ Coverage   61.06%   61.12%   +0.05%     
==========================================
  Files         103      103              
  Lines        9599     9613      +14     
==========================================
+ Hits         5862     5876      +14     
  Misses       3250     3250              
  Partials      487      487              

Flag	Coverage Δ
unittests	61.12% <90.90%> (+0.05%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pkg/runtimeproxy/server/docker/handler.go	32.17% <0.00%> (ø)
pkg/runtimeproxy/server/docker/utils.go	83.06% <100.00%> (+2.15%)	⬆️

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[ZYecho]

/lgtm

[jasonliu747]

/lgtm

[hormes]

/approve

[koordinator-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: hormes, jasonliu747

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