Standardise Mechanism for Skipping Docker Image Building

[kilo52]

When creating a new project with project-init and enabling Docker integration, the default project control code always rebuilds the Docker image. In most cases that's the desired and most convenient behaviour so that users do not have to remember to rebuild manually when something changes. When no changes are detected, a Docker image can be rebuilt very quickly since layers are cached.

However, there should be a standardised way to specify, as part of the project control code or the project's environment itself, that the (re)-building of the Docker image should be skipped. For example, this could be accomplished by defining an environment variable which is then checked by the project control code. We might introduce a common pattern for this behaviour.