Change mainContainer env handling

[prateekkhera92]

Summary

mainContainer.env in workflow controller configmap lets one add default environment variables to workflow main container. However, the current nature currently is to override these env variables, if there are env variable defined in workflow manifest file. I think, the correct behaviour should be extending it, so that default variables are preserved. However, if there is a key in env that exists in both, workflow manifest should override the default ones in mainContainer.env

What change needs making?

Use Cases

It will let us define default env variables correctly

When would you use this?

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[terrytangyuan]

Overriding is the expected behavior instead of extending.

[gyanprakash48]

@terrytangyuan whats the possible way then if we want to have a env variable that should be available to all task , but still possible to add additional env variable at workflow manifest level as well

[gyanprakash48]

i also have a case where this proposed enhancement will be perfect solution

[mio4kon]

@terrytangyuan I also found that ENV is currently fully covered, resulting in a bit high cost of use. I feel that it should be better to override the env of the corresponding key。

Is it possible to add a merge switch like this?

[terrytangyuan]

Maybe consider using a webhook to add env vars before wf controller reconciles the workflow