AppImage: build on Ubuntu trusty
We need to build on trusty to get our AppImage accepted into AppImage Hub.
I'm aware that building on trusty introduces a problem on some very up-to-date distributions, which can be worked around by exporting OPENSSL_CONF=/etc/ssl/
.
Apart from this, the docker image we build is patched so we use modern cmake, Qt, Kirigami and qxmpp and ecm.