Currently it is not possible to build the ubuntu-image snap on bionic or later. The reason for that is the libc6 mismatch between xenial (in the core snap) and all the other bits in the snap. Currently snapcraft will error out requesting libc to be added to the list of staged-packages.
We added it previously because of this error which resulted in a buildable snap for any series. Problem is that then such a snap can only be used on the platform that it has been built on. For instance, if you build the u-i classic snap on a bionic system, the snap will work fine on bionic (and cosmic actually) but die terribly on xenial - and vice-versa. This is due to the ABI mismatch between the core snap, the u-i snap contents and the libraries present on the host system.
This needs to be somehow fixed. We need to dig into which exact libraries are causing the segfaults - do we need to include more dependencies in the u-i snap? Is it actually possible to include the build-system libc in the snap and still be compatible with core16, xenial and bionic at once?
Without this fixed our CI will also be showing failures in the snap.sh test for non-xenial systems.
