ecs run: add image override option

[nwsparks]

One of the downsides of fargate is the inability to get a shell in running containers. To workaround this we have alternative versions of our containers built with ssh installed and a fairly lengthy script to launch them as one offs for temporary usage.

Having the ability to do ecs run with an image override would simplify things for us.

There is actually a pretty lengthy aws issue here discussing this, aws/containers-roadmap#187 where I think quite a few people would potentially benefit from this.

[nwsparks]

ecs exec is now a thing so could probably close this unless others think it would be useful.