Add `r_maria` and `r_mysql` images

[krlmlr]

The difference to rig-ubuntu-dbi are the different client libraries used.

• MariaDB: https://mariadb.com/docs/server/connect/programming-languages/c/install/#Installation_via_Package_Repository_(Linux)
• MySQL: https://stackoverflow.com/a/63795163/946850

See also https://github.com/r-dbi/RMariaDB/.

Same for r_postgres image: needs client libraries for Postgres.

Later, we will install provide more images with different versions of these client libraries.

This is required to test compilation of the DBI backend packages RMariaDB and RPostgres with different client libraries.

After #6.