[ecs] [request]: setting ulimits sanity check

I set the ulimits to -1 for a task definition (since ulimit accepts -1 for unlimited at the command line).
I don't think that ECS liked this - my containers won't start (so it appears).

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So maybe the UI should _not_ accept a negative number here?

Reproduce the reported ECS task-definition case with a ulimit value of -1 and inspect where the task definition or UI accepts that value. Confirm how ECS responds and determine the expected validation behavior. Done means the invalid negative value is handled consistently and the resulting behavior is covered by an appropriate check.

Written by the indexing model from the issue text.