Based on feedback, a few adjustments have been made to the identifier
requirements. Identifiers that have components that start with a number
are now allowed. This violates RFC 1035 but does not do so for dns names
practically. A total length, of 76 characters is now also enforced. This
decision was completely arbitrary but satifies the requirement for a
maximum length. Often, this is used as the maximum length of a line in
editors, so it should be a good choice.
Signed-off-by: Stephen J Day <stephen.day@docker.com>
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