Update with instructions to create docker image

Author: Andrea Pruccoli

README.md

@@ -2,4 +2,10 @@
 
 This project (based on the code provided by [Thingsboard](https://github.com/thingsboard/custom-oauth2-mapper)) serves as a custom mapper in order to map OAuth 2.0 user to Thingsboard user.
 
-This project is not meant to be run as it is, instead the docker image built from this source code is used in the project [IOT](https://github.com/connectpa/IOT).
+This project is not meant to be run as it is, instead the docker image built from this source code is used in the project [IOT](https://github.com/connectpa/IOT).
+
+To create the docker image, from the root folder launch the command:
+
+`mvn clean install -Ddockerfile.skip=false`
+
+The resulting docker image should be named like `psacr.azure.io/oauth2-mapper:latest`