Dockerfile: use gcr.io/distroless/static

[bassosimone]

As mentioned by @pboothe in #140, we want to run the ndt-server in a distroless container but, as mentioned by @stephen-soltesz, now it's early to do that.

I'm creating this issue to document this need.

[bassosimone]

Now that I understand priorities better, this is probably P3

[bassosimone]

Also, removing myself because I may not end up being the person doing that