Fix RPM build for macs

[tangledbytes]

Explain the changes

This PR attempts to fix RPM build process and make them more portable by relying on docker cp instead of volume mounts.

Testing Instructions:

Run make rpm and check if you can get the RPM inside build/rpm directory once done.

• Doc added/updated
• Tests added

[tangledbytes]

@romayalon @naveenpaul1 @shirady please pull this branch and see if this resolves the make rpm issue for you.

[romayalon]

Hey @tangledbytes
I tried it and looks like there is still an issue -

...
mkdir -p /build/rpm && docker cp noobaa-rpm-build-env:/export/. /build/rpm/
mkdir: /build: Read-only file system
make: *** [rpm] Error 1
noobaa-core % ls /build
ls: /build: No such file or directory
noobaa-core % pwd