Easy environment customization

A common issue I'm finding amongst my various toolboxes is that I need to set up my environment variables differently depending on which toolbox I'm working out of. For instance, one toolbox may have a different distribution/version of a runtime like Java or PHP, and I need to point the runtime to the correct configuration paths for that particular version of the runtime.

Similarly, as a convenience function, it would be nice to be able to have my different toolboxes have different $GIT_HOME and $KUBECONFIG settings. I often have a toolbox for each "project" I'm working on, and those projects have a bunch of specific configuration.

This functionality could be accomplished by something as simple as a per-container bootstrap script that can be customized.