Add bc to Dockerfile installs

[snario]

Some seth methods like to-wei depend on bc.

Before

node@c693e002dd7b:/src$ seth --to-wei 1 gwei
/usr/local/libexec/seth/seth---to-wei: line 18: bc: command not found

After

node@c693e002dd7b:/src$ seth --to-wei 1 gwei
1000000000

[snario]

Actually there are a few more things missing like ethabi and jshon, let me come back to this in a bit. Maybe the Docker container doesn't need to have support for seth?

[dbrock]

It's probably much better to use Nix

[dbrock]

We should probably change the Dockerfile to install everything using Nix