tests: add basic test for docker

[mvo5]

As sugested on the malinlist. Limited to classic currently but we want to expand to core once its fully working there.

[mvo5]

Loic told me that we need extra kernel modules for this to work on linode. So probably apt install linux-image-extra-virtual.

[mvo5]

I just re-tested this. Docker needs the aufs kernel module which is not available in our default virtual kernels it seems. This is why we need to install the linux-image-extra-virtual bits. When I did that manually (via spread -debug) and installed the right kernel and manually loaded the kernel module it worked for me.

[zyga]

Looks good, posted a question about running this on core. Even if we don't want to / cannot do that now it'd be nice to describe this in the test.

[zyga]

Should we also test this on core?

[mvo5]

Yes!

[mvo5]

This seems to be ok now:

$ spread -v -debug linode:ubuntu-16.04-64:tests/main/docker
2017/02/13 21:26:28 Preparing linode:ubuntu-16.04-64:tests/main/...
2017/02/13 21:27:08 Preparing linode:ubuntu-16.04-64:tests/main/docker...
2017/02/13 21:27:47 Executing linode:ubuntu-16.04-64:tests/main/docker (1/1)...
2017/02/13 21:27:50 Restoring linode:ubuntu-16.04-64:tests/main/docker...
2017/02/13 21:27:52 Restoring linode:ubuntu-16.04-64:tests/main/...
2017/02/13 21:27:57 Restoring project on linode:ubuntu-16.04-64...
2017/02/13 21:27:58 Discarding linode:ubuntu-16.04-64 (Spread-B)...
2017/02/13 21:27:58 Removing configuration from linode:ubuntu-16.04-64 (Spread-B)...
2017/02/13 21:27:59 Removing disks from linode:ubuntu-16.04-64 (Spread-B)...
2017/02/13 21:28:00 Successful tasks: 1
2017/02/13 21:28:00 Aborted tasks: 0

Would be nice of course if we could run it without manual: true