the part

obj["spec"]["template"]["spec"]["containers[0]"]

looks suspicious to me at the first glance

I haven’t tested that but I’d write:

obj["spec"]["template"]["spec"]["containers"][0]["env"] = ...

I strongly doubt something in the middle converts

containers

array so it’s accessible under

containers[X]

-like keys

If you ask how to overwrite an entry within a list in python, you can do something like that:

from typing import List, Dict

...

def overwrite_env(envs: List[Dict], name: str, new_value: str) -> bool:
  for env in envs:
    if env["name"] == name:
      env["value"] = new_value
      return True
  return False

...

def set_deployment_env_var(obj, opts):
    if obj["kind"] == "Deployment":
        envs = obj["spec"]["template"]["spec"]["containers"][0]["env"]
        overwrite_env(envs, "RELASE_DATE", "my-new-value-as-string")