use alpine version

use alpine version

diff --git a/Dockerfile b/Dockerfile
index 5ad6cc0..ea0cd98 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -1,4 +1,4 @@
-FROM ruby:2.6.0
+FROM ruby:2.6.4-alpine
 
 LABEL "name"="Publish to Rubygems"
 LABEL "version"="1.0.0"

GitHub sha: 48e0c0c9