Add configuration for preview webserver

[pestevez]

This PR adds support to configure the use of the preview webserver.

Here is a summary of the changes:

• Add dotenv package to load environment variables from a file.
• Added three variables to config the webserver:
  • One to define if the local webserver should be started. It is expected to be false when using a remote webserver.
  • One to define the host (actually, it is the protocal and hostname) of the remote webserver.
  • One to define the port of the remote webserver.
• Refactored the logic to start and stop the webserver to handle the config based on the new variables.
• Updated Preview React component to remove the hardcoded local webserver URL and replace it with the configurable one.
• Added a Dockerfile that can be used to run a Docker container with a preview webserver.
• Updated the Readme file with instructions to configure the webserver.

Note: still need to figure out why flow is failing.

Fixes #159.

[pestevez]

Thanks @nataliemt for figuring out the issue with flow!

[pestevez]

@nataliemt should be good to go now!

[nataliemt]

It looks great @pestevez, thanks!