use return statement instead of exit in is_public_ecr_logged_in (#197)

Description of changes:
* use return statement instead of exit in `is_public_ecr_logged_in` function
* tested locally that `kind-test` passes and builds docker image
*  
By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

Author: vijtrip2

scripts/lib/common.sh

@@ -131,12 +131,12 @@ is_public_ecr_logged_in() {
     local expiration_time=$(jq -r --arg url $public_ecr_url '.auths[$url].auth' ~/.docker/config.json | base64 -d | cut -d":" -f2 | base64 -d | jq -r ".expiration")
 
     # If any part of this doesn't exist, the user isn't logged in
-    [ -z "$expiration_time" ] && exit 1
+    [ -z "$expiration_time" ] && return 1
 
     local current_time=$(date +%s)
 
     # If the credentials have expired, the user isn't logged in
-    [ "$expiration_time" -lt "$current_time" ] && exit 1
+    [ "$expiration_time" -lt "$current_time" ] && return 1
 
-    exit 0
+    return 0
 }