Bump nf-core/base image in Dockerfile on each release

Each tools release should bump the version used in the `Dockerfile` to a tagged release (e.g. 1.7 for the upcoming one). 

https://github.com/nf-core/nf-co.re/issues/125#issuecomment-493038041 basically  @MaxUlysse came up with the idea. It would be wise to

- make `nf_core create` aware of this
- make `nf_core lint` aware of this

... and test this behaviour ;-) 




 