Don't accept initial / as container name

Summary:
The Docker registry allows to use /foo or foo as container name.

This is a dubious comfort feature, but it would be nice if queries
like %2Ftmp (/tmp URL encoded) doesn't have a lot of chance to
succeed: even if the API is intended to expose metadata of a
filesystem without any secret, it could theoretically be deployed
into environment where filesystem can expose secrets.

Test Plan: Browse {{URL}}/docker/registry/repository/%2Ftmp/

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D1879