The API_URLS environment variable can be used to run the Docker image with multiple urls.

Example:
docker run -p 80:8080 -e API_URLS=[{name:"First API", url:"http://firstapiurl"},{name:"Second API", url:"http://secondapiurl"}] swaggerapi/swagger-ui

* Use PORT to override nginx listen port when defined
* Add dockerignore to make build context smaller

If swagger-ui is run behind reverse proxy in path other than / accessing
bundled swagger json configured in SWAGGER_JSON env variable will fail.

Fix that by changing REL_PATH prefix from `/` to `./`

Fix "sed" regex in docker-run.sh to use the VALIDATOR_URL environment variable.
Don't use it in quotes if it's defined to "null" or "undefined".