revert part of changes done in #443 due to file mount issues on windows

[Emilgardis]

Without this, cross on windows with docker for windows errors on

cross build --target armv7-unknown-linux-gnueabihf
docker: Error response from daemon: Mount denied:
The source path "\working\dir:/C:\working\dir:Z"
too many colons.
See 'docker run --help'.

If we somehow fixed that error, we'd then get

cross build --target armv7-unknown-linux-gnueabihf
docker: Error response from daemon: the working directory 'C:\working\dir' is invalid, it needs to be an absolute path.
See 'docker run --help'.

this pr simply reverts the project structure changes done in #443 to try and resolve that.

I'm unclear if this functionality is needed for the build env volumes.

cc @reitermarkus

[reitermarkus]

I think this will break volumes given as relative paths.

[Emilgardis]

I want to get this added. I don't think relative paths are important for this feature.

Correction: relative paths like ./path I think are important, but not ../path. As such, some logic should probably be available to consumers to mount paths within ../**/path to docker /path

[Emilgardis]

To get this fixed, I've been thinking of instead having it mount like this, it should be configurable.
This also brings an added previous benefit of not leaking the path the user used to build, i.e leaking usernames etc.

[Emilgardis]

closing in favor of new pr #665