No information on providing docker credentials as environment variables in the document

[isuruuy429]

Description:
Doc: https://ei.docs.wso2.com/en/latest/ballerina-integrator/develop/deploy-on-docker/

There is no information in providing the docker credentials as environment variables in the document. When it was executed as per the instructions in the doc it gives the following error.

error: isuruuyanage/healthcare_service:0.1.0::healthcare_service.bal:36:1: error resolving value: DOCKER_USERNAME is not available as an environment variable or a system property.