add share ipc and fix dconf permissions

[nedrichards]

If you use X11 you should also have --share=ipc in finish args. From the docs:

", --share=ipc means that the sandbox shares IPC namespace with the host. This is not necessarily required, but without it the X shared memory extension will not work, which is very bad for X performance."

This should fix flatpak/freedesktop-sdk-images#66

Also, the dconf permissions specified weren't 'enough' for it to work more generally.

[nedrichards]

bot, build

[thiblahute]

Thanks :-)