No concurrent builds

For safety reasons, Quay.io only performs one build at a time per repository. This is because if one build was started shortly after another and the second build finished first, the initial build would then clobber the binary assets created by this build process.

Quay Enterprise may solve this, but only from the perspective that the builds are on dedicated hardware which may run faster. In Quay Enterprise, we still only run a single build on a single repository for the same safety reasons mentioned above.