To update any container you will have to make use of : az container create with options
(See the below document along with the commands)
https://learn.microsoft.com/en-us/azure/container-instances/container-instances-update
To create the container with environment variables:
Also there are some properties which can't be updated but that needs container to be re-created:
So for Azure Container Instances, those are the only options which are provided as of now.
Kindly explore those options !!
Regards,
Shiva.