LAUNCH_URL parameter can't be used for file:/// or data: URI's

[tim-littlefair]

How to reproduce:

Add the lines:

    environment:
      - LAUNCH_URL=file:///home/chromium/index.html

or

    environment:
      - LAUNCH_URL=data:text/html;charset=utf-8;base64,PGh0bWw+SGVsbG8gV29ybGQ8L2h0bWw+

to the original docker-compose.yml file and push the block to a device.

In both cases, src/server.js will prepend "http:" before the requested URI, the URI will fail to address a valid resource and Chromium will display a Google search page.

NB The data URI in the second example was generated on https://dopiaza.org/goodies/data-uri-generator/.
It encodes a URI containing mime-type text/html and data string "Hello World".

Being able to configure launch URI's of this type will make it possible to give a neater UX when starting up an integration containing the browser block and another container providing the web server the browser displays (expecially if the second container can't offer and instantaneous startup serving valid data).