I am talking about either, because the GP post was about "containerizing a build environment": you need your project built to either run it in CI or locally.
Why would it be slow?
It needs to be rebuilt? (on a fast moving project with mid-sized or large team, you'll get dependency or Dockerfile changes frequently)
It needs to restart a bunch of dependent services?
Container itself is slow to initialize?
Caching of Docker layers is tricky, silly (you re-arrange a single command line and poof, it's invalidated, including all the layers after) and hard to make the most of.
If you can't get a single test running in <1s, you are never going to get a full test suite running in a couple of seconds, and never be able to do an urgent deploy in <30s.
Why would it be slow?
It needs to be rebuilt? (on a fast moving project with mid-sized or large team, you'll get dependency or Dockerfile changes frequently)
It needs to restart a bunch of dependent services?
Container itself is slow to initialize?
Caching of Docker layers is tricky, silly (you re-arrange a single command line and poof, it's invalidated, including all the layers after) and hard to make the most of.
If you can't get a single test running in <1s, you are never going to get a full test suite running in a couple of seconds, and never be able to do an urgent deploy in <30s.