update Oracle setup docs

[simonpcouch]

Some notes-to-self after an afternoon trying to make this happen... the following from Jim remains true:

https://github.com/r-dbi/odbc/blob/e8b68fa5de44bf3d3cd7cfedb204bf8f7c260f16/vignettes/develop.Rmd#L203

😝

Oracle Cloud Free as a quick setup to test against is a no-go: requires a credit card and, even with that, had some opaque account setup issues that I wasn't able to resolve.

Tried with colima and then reverted to the docker host, but think I'll need to transition back to trying colima again.