when building haddock under a docker image it suggests the wrong install option for hscolour

it says:

> Warning: haddock not generating hyperlinked sources because 'HsColour' not
> found on PATH (use 'stack install hscolour' to install).

but that command, `stack install hscolour`, won't work if you've invoked stack with `stack --docker --docker-image ... haddock`

stack version:

```
stack --version
Version 1.2.1, Git revision 8991addb0ad49fd6ca1a7c66b71303389ce2f5a1 x86_64 hpack-0.14.1
```

this is very minor. could be fixed by perhaps recalling the command that was used to invoke stack?