Docker module 'env' parameter supports reading values from a file.

From `docker run` help:
```
--env-file=[]              Read in a line delimited file of environment variables
```

This adds the same capability to the Docker module. Following this PR, "env" parameter can be a string of the format "@/path/to/file" and the module will interpret the environment variables and feed them to the container.

Example file `environment.env`:
```
SOME_VAR=value
#
# Comments
#
OTHER_VAR=other value
```