Remove App Docker Gradle plugin

• Type: Task
• Status: Closed
• Priority: Minor
• Resolution: Fixed
• Fix Version/s: 7.0.x, 7.1.x, 7.2.x
• Component/s: Dev Tools
• Labels: None
• Type of Documentation: Developer: Reference
• Git Pull Request: https://github.com/sez11a/liferay-docs/pull/4448 , https://github.com/sez11a/liferay-docs/pull/4449 , https://github.com/sez11a/liferay-docs/pull/4450

This is an internal plugin that is no longer recommended for external usage. Remove the official doc for it.