[ECS] Define container dependencies

[izeau]

Is your feature request related to a problem? Please describe.
Sometimes a sidecar needs to be started before the actual main container, sometimes it’s the other way around, sometimes a sidecar requires another sidecar, etc. AWS provides a dependsOn parameter, but it does not look like it is exposed by Waypoint.

Describe the solution you'd like
A way to define container dependencies for both the main container and for the sidecars.

Describe alternatives you've considered
I’m considering using Waypoint but this missing feature is annoying.

Explain any additional use-cases
None off the top of my head

Additional context
None at the time, maybe later