Add MIRRORD_IMPERSONATED_CONTAINER_NAME var

[camerondurham]

Details

This CR adds an optional MIRRORD_IMPERSONATED_CONTAINER_NAME to specify container name within pod for #100.

This doesn't implement ignoring linkerd/envoy pods yet. Can iterate on this draft based on feedback. I suppose the best way to ignore containers like this is to filter out common container names for linkerd/envoy within a pod?

A couple things that are missing, I'll convert to draft if these are blockers.

1. add ability to set this variable in VSCode extension
2. Ignore linkerd/envoy container names

Testing

These are low effort tests and not really best practice since they won't be in CI. I can try this weekend adding a mock failure run to sanity.rs.

1. local GH action run manually setting MIRRORD_IMPERSONATED_CONTAINER_NAME=test (link)
2. local GH action run not setting MIRRORD_IMPERSONATED_CONTAINER_NAME (link)
3. local GH action sanity check failed when setting MIRRORD_IMPERSONATED_CONTAINER_NAME=testgarbage (link)

Example failure:

https://github.com/camerondurham/mirrord/runs/7167505799?check_suite_focus=true#step:15:76

stderr 7282: thread '<unnamed>' panicked at 'called `Result::unwrap()` on an `Err` value: no container named testgarbage found in namespace=default, pod=http-echo-gw7u7b2-59cdf9cf46-pfqwp', mirrord-layer/src/pod_api.rs:49:18

Thanks!

[aviramha]

Awesome! This looks great. I left two comments, one I consider a nitpick and the other is UX but small so would be required to get merged.

[aviramha]

Great, Thank you.