sharness: disable t0300-docker-image on Circle CI

The actual tests for the IPFS daemon within the image,
which were added in the previous commits, made Circle CI unhappy.

Circle CI runs an old version of Docker
which still uses LXC instead of libcontainer.
The existing lxc-attach hack is testament to that.

We might be able to get it work [1][2]
but it's really not worth it at the moment,
and Circle CI isn't a good option for Docker things, right now.

[1] https://jpetazzo.github.io/2014/03/23/lxc-attach-nsinit-nsenter-docker-0-9/
[2] https://github.com/jpetazzo/nsenter



License: MIT
Signed-off-by: Lars Gierth <larsg@systemli.org>