#1852 Remote API documentation incorrect regarding private registries

Issues addressed:

- Remote API doco refers to "registry" parameter which is not supported
- Neglects to mention "tag" parameter which is supported
- Offers no guidance on pushing to private registries

Changes made:

- As directed in review comments, `registry` removed from all api versions (1.0-1.14)
- Have added documentation for `tag` parameter and guidance on private registries for versions 1.10-1.14 (I have only validated this on .10)

Docker-DCO-1.1-Signed-off-by: Lachlan Coote <lachlan.t.coote@gmail.com> (github: lcoote)
Fix example of relative WORKDIR

Docker-DCO-1.1-Signed-off-by: Alexandr Morozov <lk4d4math@gmail.com> (github: LK4D4)
Merge pull request #7363 from lcoote/master

 #1852 Remote API documentation incorrect regarding private registries
Merge pull request #7391 from LK4D4/fix_relative_workdir_example

Fix example of relative WORKDIR