Specify Docker for Mac use of osxkeychain impact
Specify Docker for Mac use of osxkeychain impact

manifest-tool can't use the osxkeychain, so clear up how authentication
is handled in the README documentation, and add an important note about
Docker for Mac since 17.03 release.

Signed-off-by: Phil Estes <dbac1f450dc8326caa7ba104b384a0eca642250c@gmail.com>

estesp/manifest-tool,estesp/manifest-tool