Use onlyIf for build Docker image task execution

[jasontedor]

This commit switches to using an onlyIf to determine if a build Docker image task execution should occur. This is preferred since it means that the determination is performed at task execution time, rather than during configuration.

Relates #53936

[elasticmachine]

Pinging @elastic/es-core-infra (:Core/Infra/Build)

[mark-vieira]

This is preferred since it means that the determination is performed at task execution time, rather than during configuration.

For clarity, this is one reason, which in this particular case is somewhat irrelevant since determining the current architecture is neither costly nor likely to change during execution.

To me, the primary reason for using onlyIf to model toggling of tasks is that onlyIf conditions can be composed. That is, if I have multiple plugins that both add an onlyIf condition to a task, the result is the combination of the two (task will execute only if both are true). However, it's very easy for plugins to clobber another plugin's enabled predicate without realizing it.

Just thought I'd provide some more context here for my insistence on preferring onlyIf over enabled. Thank you for your continued cooperation 😉

[mark-vieira]

👍

[jasontedor]

Sir, yes sir! Thanks for the explanation @mark-vieira. 🙂