docker: fix dockerfile warnings

[jaydee-coder]

The following warnings were emitted when running make docker-test:

 2 warnings found (use docker --debug to expand):
 - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 11)
 - JSONArgsRecommended: JSON arguments recommended for CMD to prevent unintended behavior related to OS signals (line 12)

This change fixes both of these 2 trivial warnings.

I added the bash -i option just in case some of the tests/tools care about whether the shell is an interactive shell or not.
Even though the tests passed successfully without that flag, I thought I'd add it just in case. It only added 0.5s to the total test time (64.8s -> 65.3s) so it's probably good to keep. Please let me know if you'd want me to remove it.

[jaydee-coder]

Actually timed it again, and now the tests take 63.2s, so the change is in the noise and probably doesn't affect timing.

[junegunn]

Thanks!