diff --git a/01-docker-images/demo/Gemfile b/01-docker-images/demo/Gemfile
index 4bcec7e..b427755 100644
--- a/01-docker-images/demo/Gemfile
+++ b/01-docker-images/demo/Gemfile
@@ -8,7 +8,7 @@ gem 'rails', '~> 5.2.1'
 # Use sqlite3 as the database for Active Record
 gem 'sqlite3', '~> 1.3.6', '< 1.4'
 # Use Puma as the app server
-gem 'puma', '~> 3.12'
+gem 'puma', '~> 4.3'
 # Use SCSS for stylesheets
 gem 'sass-rails', '~> 5.0'
 # Use Uglifier as compressor for JavaScript assets
diff --git a/01-docker-images/demo/Gemfile.lock b/01-docker-images/demo/Gemfile.lock
index 40de027..b73d37a 100644
--- a/01-docker-images/demo/Gemfile.lock
+++ b/01-docker-images/demo/Gemfile.lock
@@ -97,7 +97,9 @@ GEM
     marcel (0.3.3)
       mimemagic (~> 0.3.2)
     method_source (0.9.2)
-    mimemagic (0.3.3)
+    mimemagic (0.3.10)
+      nokogiri (~> 1)
+      rake
     mini_mime (1.0.1)
     mini_portile2 (2.4.0)
     minitest (5.11.3)
@@ -107,7 +109,8 @@ GEM
     nokogiri (1.10.9)
       mini_portile2 (~> 2.4.0)
     public_suffix (3.0.3)
-    puma (3.12.4)
+    puma (4.3.12)
+      nio4r (~> 2.0)
     rack (2.0.8)
     rack-test (1.1.0)
       rack (>= 1.0, < 3)
@@ -201,7 +204,7 @@ DEPENDENCIES
   coffee-rails (~> 4.2)
   jbuilder (~> 2.5)
   listen (>= 3.0.5, < 3.2)
-  puma (~> 3.12)
+  puma (~> 4.3)
   rails (~> 5.2.1)
   sass-rails (~> 5.0)
   selenium-webdriver