The Dockerfile should be defined as an InputFile

[dsmiley]

The task "docker" supports a custom "Dockerfile" at the project root. However, it's apparent that the code for this plugin does not declare to Gradle that the Dockerfile is one of the task's "inputs". Because of this problem, when I change my Dockerfile, running the docker task directly or indirectly are short-circuited by Gradle's incremental build feature, since it thinks there have been no changes that are pertinent to this task.

https://docs.gradle.org/current/userguide/more_about_tasks.html#sec:up_to_date_checks

I've worked around this by specifying

tasks.docker {
  inputs.file('Dockerfile')
}