This sets the default feature to sqlite, so that it will correctly
compile "out of the box". Users can still choose different backend by
running:
```
cargo build --no-default-features --features <backend>
```
This changes the healthcheck to use `sh` instead of bash, that is absent
from some image versions. (like alpine)
It also removes `*mariadb*` packages from runtime image of sqlite images
as these shouldn't be required.
Miro Prasil
Daniel García
H3npi
Felix Bartels