Hi,
first of all, using Docker should be a good choice. You could leave the configured scm home variable the way it is and simply mount your old scm home to /var/lib/scm. Your old repository folders should be mounted to the exact old paths in docker (that is /opt/svn and /opt/git), because SCM-Manager will look for them in these places during the migration. If you choose INLINE for all repositories, these paths will be kept and only the file structure inside the paths will be adapted to the new v2 layout. If you do not mind the new location of the repositories, you can also choose MOVE or COPY to end up with only one mounted directory (see here for the different strategies).
Keep in mind to make a backup first 
We would love to hear from you how it’s going.
René