Ensure container names start with a-zA-Z0-9

Closes #8012.

Docker-DCO-1.1-Signed-off-by: Jessica Frazelle <jess@docker.com> (github: jfrazelle)
Merge pull request #8014 from jfrazelle/8012-prevent-container-names-starting-dash

Ensure container names start with a-zA-Z0-9