No main branch for version 5.10-12

[jarrodback]

Can't pull the dockerfile for 5.10, 5.11, 5.12 as the branches are missing: https://github.com/danlynn/ember-cli/blob/5.12.0/Dockerfile

Works for 6.1.0: https://github.com/danlynn/ember-cli/blob/6.1.0/Dockerfile