Insert placeholder in place of removed environment variables

[brianredbeard]

When removing the environment variables from Dockerfiles (docker.go#L61), it isn't clear that there were ever environment variables there to begin with. It would be good to potentially inject a single variable to be used as a comment (the JSON format doesn't allow comments) informing the user that the variables were removed.

Alternately, replacing the values with something to the effect of scrubbed by mayday would allow the user to see that the container was instantiated with the correct variable names even if they don't know the values.

Affects #32

[tschuy]

I've pushed something to this effect in #32.