You absolutely don't need to be a cloud person to use containers. For example I use lxc on my laptop to keep different projects separate and test new software.
Here is a general tutorial in anyone wants to test lxc:
100% agree. Most developers don't even bother testing code that requires DBs and other external services because it's a pain to keep test environments up and running so a lot of the most important and buggy code slips through the cracks.
Then you have the developers who do test but just make heavy use of mocking libs and other tricks which aren't helpful in the long run since the mocks don't actually do anything.
But setting up a docker compose with the whole stack - completely reset each time the tests are run - makes integration tests so much more valuable. And usually it's not a huge deal to set up the plumbing since the docker files are often already created for production.
This is less true these days (and on macs that had to use the docker machine "hack" it was barely ever true, per say) -- rootless containers are on the way thanks to user namespaces. For example LXC can run fully rootless containers that act more like VMs themselves (as in they will have systemd as pid 1 inside) -- kernel support, user namespaces, and File systems in Userspace (FUSE) make this possible.
IMO One of the biggest benefits to running containers is actually E2E tests -- I don't see it done as often as I should, but it has become drastically easier to run an entire postgres instance for a single local E2E test runs -- I do this on almost every project I start now, I set up E2E tests that spin up the actual world I expect in production (so all the backing services, at the version they will run at), and interact with my application -- this is a huge step forward compared to a huge wiki that has "how to set up the local test VM" -- you can spin up and shutdown these services so fast that you can use them ephemerally (I do) without much worry.