Use DOCKER_HOST env to search for docker before searching for a hardcode docker socket

[mscherer]

Fix #757

[mscherer]

I haven't test the PR yet, might not have time before the weekend, so that's more a proposal.

[wxiaoguang]

The DOCKER_HOST could be a URL:

https://docs.docker.com/engine/security/protect-access/

$ export DOCKER_HOST=ssh://docker-user@host1.example.com
$ docker info

I think as long as the environment exists, we do not to check the existence again.

[mscherer]

Ok so more like this ?

[wxiaoguang]

Now it looks good 😊

[codecov-commenter]

Codecov Report

Merging #763 (a3125a0) into master (5f79739) will not change coverage.
The diff coverage is 100.00%.

❗ Current head a3125a0 differs from pull request most recent head 43b9bda. Consider uploading reports for the commit 43b9bda to get more accurate results

@@           Coverage Diff           @@
##           master     #763   +/-   ##
=======================================
  Coverage   52.23%   52.23%           
=======================================
  Files          86       86           
  Lines        5940     5940           
=======================================
  Hits         3103     3103           
  Misses       2665     2665           
  Partials      172      172           

Impacted Files	Coverage Δ
server/shared/procBuilder.go	86.14% <100.00%> (ø)

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