I looked into this more, and I learned that the “classic” confinement mode I am using for snap does not let it use any of the packages installed in the snap. In classic mode, it is little more than repackaging the portable zip. Therefore, it does require installing the system packages. I am using classic confinement because the export jobs will not start in “strict” confinement, and I just don’t have the time or experience on how to debug something running in strict confinement like that. The difference in confinement - besides library dependency resolution - is that strict is an execution sandbox.
I also discovered that for some time - probably about a year - Shotcut Linux builds have included some unnecessary dependencies such as gstreamer. I have made a new v17.11.04 build to fix a MP3 crash bug that reduces these dependencies. You should not need SDL 1.2 or gstreamer. You should also not need libsox2 as my portable build includes it, but libsox has a dependency on libltdl, which is not included. I figured out some of these while working on a snap in strict confinement with latest build. There are some other dependencies not available in a clean install of Ubuntu 17.10, for example, that I will bundle in next month’s build. The reason some dependencies like SDL2 and Jack are not bundled is because the version in my build agent either pulls in a big sub-tree of dependencies that I do not want to include or because it does not play nice on some Linux OSes.
There will be a new snap soon for v17.11 with fewer dependencies, and I will test an updated apt-get to satisfy the new dependencies.