Remove '-f' option from tag - add note for login.

docker/build/go-agent/README.md

@@ -36,9 +36,10 @@ necessary.
     - ```make docker.test.go-agent```
 * Log docker in to AWS
     - ```sh -c `aws ecr get-login --region us-east-1` ```
+        - You might need to remove the `-e` option returned by that command in order to successfully login.
 * Tag image 
-    - ```docker tag -f <image_id> ############.dkr.ecr.us-east-1.amazonaws.com/prod-tools-goagent:latest```
-    - ```docker tag -f <image_id> ############.dkr.ecr.us-east-1.amazonaws.com/prod-tools-goagent:<version_number>```
+    - ```docker tag <image_id> ############.dkr.ecr.us-east-1.amazonaws.com/prod-tools-goagent:latest```
+    - ```docker tag <image_id> ############.dkr.ecr.us-east-1.amazonaws.com/prod-tools-goagent:<version_number>```
 * upload: 
     - ```docker push ############.dkr.ecr.us-east-1.amazonaws.com/edx/release-pipeline/prod-tools-goagent:latest```
     - ```docker push ############.dkr.ecr.us-east-1.amazonaws.com/edx/release-pipeline/prod-tools-goagent:<version_number>```
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