Settings.js: support configuration via environment variables.

All the configuration values can be read from environment variables using the
syntax "${ENV_VAR_NAME}".
This is useful, for example, when running in a Docker container.

EXAMPLE:
   "port":     "${PORT}"
   "minify":   "${MINIFY}"
   "skinName": "${SKIN_NAME}"

Would read the configuration values for those items from the environment
variables PORT, MINIFY and SKIN_NAME.

REMARKS:
Please note that a variable substitution always needs to be quoted.
   "port":   9001,          <-- Literal values. When not using substitution,
   "minify": false              only strings must be quoted: booleans and
   "skin":   "colibris"         numbers must not.

   "port":   ${PORT}        <-- ERROR: this is not valid json
   "minify": ${MINIFY}
   "skin":   ${SKIN_NAME}

   "port":   "${PORT}"      <-- CORRECT: if you want to use a variable
   "minify": "${MINIFY}"        substitution, put quotes around its name,
   "skin":   "${SKIN_NAME}"     even if the required value is a number or a
                                boolean.
                                Etherpad will take care of rewriting it to
                                the proper type if necessary.

Resolves #3543

docker/README.md

@@ -14,6 +14,8 @@ cp ../settings.json.template settings.json
 [ further edit your settings.json as needed]
 ```
 
+**Each configuration parameter can also be set via an environment variable**, using the syntax `"${ENV_VAR_NAME}"`. For details, refer to `settings.json.template`.
+
 Build the version you prefer:
 ```bash
 # builds latest development version