Allow for additional "local .dazelrc"

[markkohdev]

Firstly thanks for building this! It makes setting people up to develop a bazel repo much easier.

The one problem I'm having is that we need to mount secrets from our machines into the dazel container (using DAZEL_VOLUMES) but since these secrets are located on different locations in each developers' machine (typically somewhere within their $HOME directory) we each need to change the value to this locally.

I'd like to have the ability to override values in the standard .dazelrc with custom values from a separate .dazelrc.local file which we wouldn't source control. I'll probably put in a PR for this but it seems like it should be pretty straightforward to implement.