Add tip about running online openapi-generator via Docker

Add tip about running online openapi-generator via Docker

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: master, 3.4.x, 4.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

(details of the change, additional tests that have been done, reference to the issue for tracking, etc)

docs/online-openapi-generator.md

@@ -9,6 +9,8 @@ cd modules/openapi-generator-online
 mvn spring-boot:run
 ```
 
+:bulb: The online openapi-generator can be run via [Docker](https://github.com/OpenAPITools/openapi-generator#16---docker) as well.
+
 For example, to generate Ruby API client, simply send the following HTTP request using curl:
 ```sh
 curl -X POST -H "content-type:application/json" -d '{"openAPIUrl":"https://raw.githubusercontent.com/openapitools/openapi-generator/master/modules/openapi-generator/src/test/resources/2_0/petstore.yaml"}' http://localhost:8080/api/gen/clients/ruby