kebab-case for action inputs, fix README, fix comments in retry tests

Signed-off-by: Fedor Dikarev <fedor.dikarev@gmail.com>

action.yml

@@ -24,14 +24,14 @@ inputs:
     description: 'Log out from the Docker registry at the end of a job'
     default: 'true'
     required: false
-  http_codes_to_retry:
+  http-codes-to-retry:
     description: 'Comma separated list of HTTP error codes we want to retry'
     default: '408,500,502,504'
-  max_attempts:
+  max-attempts:
     description: 'Overall maximum number of attempts we will make trying to login'
     default: '1'
     required: false
-  retry_timeout:
+  retry-timeout:
     description: 'Timeout between retries, in seconds'
     default: '15'
     required: false