Revert container recipes to pull master

[Lestropie]

As per comment in #2356.

The theory here is that:

• If someone checks out the code in 3.0.3, and then builds a container using one of the recipe files, during the building of that container it should download and compile the MRtrix3 code specifically at tag 3.0.3, regardless of how master may have changed after that point;

• If someone checks out the code in master, and then builds a container using one of the recipe files, during the building of that container it should download and compile whatever the MRtrix3 master branch is pointing to at that point in time, regardless of what tags may or may not have been added.
  (Better would be if, during container build, it would check out the MRtrix3 code at the commit hash corresponding to the code version of the recipe that is dictating the build process; but I have not yet thought about how such could be achieved).