[docs] Add documentation for GPU w/ docker-engine

[CecileRobertMichon]

What this PR does / why we need it: GPU nodes require docker-engine. Add documentation for the current workaround implementation. Follow-up for #4262.

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #

Special notes for your reviewer:

If applicable:

• [ ] documentation
• [ ] unit tests
• [ ] tested backward compatibility (ie. deploy with previous version, upgrade with this branch)

Release note:

[CecileRobertMichon]

/hold incoming PR to change the implementation

4270

[codecov[bot]]

Codecov Report

Merging #4268 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4268   +/-   ##
=======================================
  Coverage   50.49%   50.49%           
=======================================
  Files         109      109           
  Lines       17159    17159           
=======================================
  Hits         8664     8664           
  Misses       7713     7713           
  Partials      782      782

[jackfrancis]

/lgtm