make the port mapping deprecation error message more obvious see 4075

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@home.org.au> (github: SvenDowideit)
Merge pull request #4418 from SvenDowideit/mention-expose-in-port-mapping-deprecation

make the port mapping deprecation error message more obvious