remove addString and replace Tail

Docker-DCO-1.1-Signed-off-by: Victor Vieux <vieux@docker.com> (github: vieux)
Minor fixups to the Docs README

Docker-DCO-1.1-Signed-off-by: James Turnbull <james@lovedthanlost.net> (github: jamtur01)
Updated a bunch of formatting in the docs/sources/use files

Docker-DCO-1.1-Signed-off-by: James Turnbull <james@lovedthanlost.net> (github: jamtur01)
Merge pull request #5801 from jamtur01/fixups

Minor fixups to the Docs README
Add some more specific help for Dockerfile build contexts.

I've copy and pasted the error message so its googlable.

Docker-DCO-1.1-Signed-off-by: SvenDowideit <SvenDowideit@home.org.au> (github: SvenDowideit)
Merge pull request #5800 from jamtur01/paras

Updated a bunch of formatting in the docs/sources/use files
Merge pull request #5706 from vieux/remove_add_string
Merge pull request #5762 from SvenDowideit/pr_out_add_some_more_specific_help_for_dockerfile_build_contexts

Add some more specific help for Dockerfile build contexts.