Configuring custom images easily

[amotl]

Dear @vangheem and @masipcat,

for adding Mosquitto (#15), we tried to use the fastpath option to configure custom images [1] first. However, we found these problems when doing it like outlined within [2]:

1. images.configure() will not create a new slot within images.settings, so we added images.settings['mosquitto'] = {} before.
2. After we did that, we found images.configure() will also not honor the options kwarg.
3. An any case, just adding a service definition to images.settings will not be sufficient. We also had to add a minimal image class derived from BaseImage [3] as well as the fixture function itself [4].

While we have been able to work around these issues, we might think about improving this procedure to get closer to DWIM.

With kind regards,
Andreas.

[1] https://github.com/guillotinaweb/pytest-docker-fixtures#configuring-custom-images
[2] https://github.com/hiveeyes/terkin-datalogger/blob/ccaa56d6/test/util/mosquitto.py#L8-L19
[3] https://github.com/hiveeyes/terkin-datalogger/blob/ccaa56d6/test/util/mosquitto.py#L32-L40
[4] https://github.com/hiveeyes/terkin-datalogger/blob/ccaa56d6/test/util/mosquitto.py#L43-L49