Restrict length of repository names

The registry restricts the length of repository names
to <64 characters. If a repository has a namespace (i.e. user), this
only applies to the name right of the /.

For instance both of the following would be considered 64 characters:

 * 123456789012345678901234567890123456789012345678901234567890123
 * ewindisch/123456789012345678901234567890123456789012345678901234567890123

Signed-off-by: Eric Windisch <eric@windisch.us>