Add HOSTNAME to env by default for pod containers

[estesp]

To match expectations of users coming from Docker engine runtime, add
the HOSTNAME to the environment of new containers in a pod.

Signed-off-by: Phil Estes [email protected]

[Random-Liu]

/lgtm

We may want to cherry-pick this to fix containerd 1.1.x.

[estesp]

I'm happy to add it back on that branch as well

[Random-Liu]

It looks like this PR broke the e2e test. The reason is:

1. cri-containerd e2e test started failing after this PR merged: https://k8s-testgrid.appspot.com/sig-node-containerd#e2e-gci
2. containerd e2e test is fine (using old cri, which doesn't have this change), so kubernetes change can be mostly ruled out: https://k8s-testgrid.appspot.com/sig-node-containerd#containerd-e2e-gci

I'm going to revert this change for now, and see whether it brings back the test back to normal.

• If it does, we need to revert the changes in supported branches, and investigate why this breaks the test.
• If it doesn't, we can submit this PR again and find the actual root cause.