If this is just running tests locally, it seems deeply flawed - e.g. the tests would work even if I forgot to commit new files.

OTOH if starting a new container, pulling my branch and then doing everything, it's definitely as good as running on remote CI because it's basically replicating the same behaviour. And it would likely still be much faster since:

* CI machines are underpowered compared to dev laptops/desktops. e.g. our CI builds run on a container with 4 vCPUs and 16GB RAM. In contrast my laptop has 16 cores and 48 GB RAM

* docker pull itself takes a relatively long time on CI. If I'm running it locally it would just use the cached layers