Allow custom plugin images using dokkurc variables + document dokkurc

Per discussion in #31 I've added support for custom database images. I've also added a section to the README which somewhat explains what is `dokkurc` about and also points to currently used images, but it is far from complete.
Originally I was interested only in PostgreSQL and Redis customization, but while I was at it, I've also edited MariaDB and MongoDB. Feel free to squash or cherry-pick any commit.