Update ruby to 2.7

Update ruby to 2.7

diff --git a/Dockerfile b/Dockerfile
index 53feb0d..81577bf 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -1,4 +1,4 @@
-FROM ruby:2.6.6
+FROM ruby:2.7
 
 LABEL "name"="Publish to Rubygems"
 LABEL "version"="1.0.0"

GitHub sha: ef23d53635d7688d5e42a6f19465f4c8e3e4bb7f

This commit appears in #2 which was approved by davidtaylorhq. It was merged by CvX.