mount /var/run/docker.sock for --use-api-socket

[ndeloof]

- What I did

s.dockerCli.DockerEndpoint().Host returns the engine URL as seen by client, which is only relevant an Unix/MacOS with a local docker engine. Waiting for engine to expose this information in some API, or implement this feature on its own, hard-coding /var/run/docker.sock offer a more reliable path for engine socket to be mounted in container, while not 100% guaranteed to succeed.

- How I did it

hard-coded path

- How to verify it

- Human readable description for the release notes

Fix `--use-api-socket` not working correctly when targeting a remote daemon.

- A picture of a cute animal (not mandatory but encouraged)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 54.83%. Comparing base (7cbee73) to head (2ba7cb8).

Additional details and impacted files

@@ Coverage Diff @@## master #6157 +/- ##
=======================================
Coverage 54.82% 54.83% =======================================
Files 364 364 Lines 30470 30467 -3 =======================================
Hits 16706 16706 + Misses 12789 12786 -3 
Partials 975 975 

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[thaJeztah]

LGTM