Demo project for spring-projects/spring-boot#29756

To reproduce the issue, execute the following steps in order:

1. mvn clean package
   Outcome: the image is built correctly, but not pushed.
   Also, the following logs are shown:

   [WARNING] The property 'docker.credentials.username' is not set or it's blank, therefore no image will be pushed.
   [WARNING] The property 'docker.credentials.password' is not set or it's blank, therefore no image will be pushed.

2. mvn clean package -Ddocker.credentials.username=something
   Outcome: the following logs are shown

   [WARNING] The property 'docker.credentials.password' is not set or it's blank, therefore no image will be pushed.

   and the build failing with

   [ERROR] Failed to execute goal org.springframework.boot:spring-boot-maven-plugin:2.6.3:build-image (build-image) on project demo: Execution build-image of goal org.springframework.boot:spring-boot-maven-plugin:2.6.3:build-image failed: Invalid Docker publish registry configuration, either token or username/password must be provided