Problem statement

This is a followup to #21142 to discuss the idea of adding a way to specify a default "alive probe" for all containers started from a particular image.

Proposal

Introduce a new Dockerfile instruction to specify the probe. The (yet to be used) support for instructions options could be leveraged to model the design proposed for the CLI.

• An image would optionally have a single probe definition, and any new occurrence of that instruction (either in the same Dockerfile, or in a child image) would override the previous one.
• The default probe could be overridden through the command-line.

Open questions:

• Ping @tianon: what's your opinion on that in the context of official images?

Ping @crosbymichael @tonistiigi @mgoelzer @aluzzardi @ehazlett