Keep env variables out of the command line

Environment variables are commonly used instead of program arguments to pass passwords and other secrets to a process because they don't show up in the process list. But currently docker sets the environment variables of the container by passing them as arguments to docker-init and so you can't really use them to send passwords.

This pull request changes that so the environment variables are instead written to a file and bind mounted into the container at /.dockerenv

Also included is a change to the run command so that you can say you want to pass an environment variable to the container and still keep the actual value out of the command line.
```
PASSWORD=test docker run -e PASSWORD ubuntu env
```