Add new fields to env.sh in contributing docs (#2121)

(cherry picked from commit 767dbdc)

Author: vishalbollu

CONTRIBUTING.md

@@ -134,6 +134,11 @@ export AWS_ACCOUNT_ID="***"  # you can find your account ID in the AWS web conso
 export AWS_REGION="***"  # you can use any AWS region you'd like, e.g. "us-west-2"
 export AWS_ACCESS_KEY_ID="***"  # alternatively, you can remove this to use the default credentials chain on your machine
 export AWS_SECRET_ACCESS_KEY="***"  # alternatively, you can remove this to use the default credentials chain on your machine
+export DEFAULT_USER_ARN="arn:aws:iam::<ACCOUNT_ID>:<AWS IAM ENTITY>" # (e.g. arn:aws-us-gov:iam::123456789:user/foo)
+
+# If you are working on govcloud, you will need to set credentials from a public (non govcloud) account to access inferentia ECR
+# export NORMAL_REGION_AWS_ACCESS_KEY_ID="***"
+# export NORMAL_REGION_AWS_SECRET_ACCESS_KEY="***"
 
 # export NUM_BUILD_PROCS=2  # optional; can be >2 if you have enough memory
 ```