Add environment variables to control cors configuration in docker images

[eladav]

The image uses the cors package from npm. The package is really configurable, check out cors on npm. We need to think which configurations make sense and what is the best way to allow people to control them. If #58 is done before this it might affect the solution to this issue.

[Yshayy]

Is there any reason to add more than "allowed origins"? I'd assume that browser apps should only fetch packages/components. Maybe even the default should be "*", since it's very similar to how browser fetch scripts/components from works from different domains.

[eladav]

Using * as the default definitely makes sense. I guess that allowed origins are the only thing that makes sense to configure. I’d say that most users indeed will go with a catch-all configuration. Bottom line: I think you’re right :)