diff --git a/.github/workflows/ci.yaml b/.github/workflows/ci.yaml
index 3a3ca77f0..08452f2cb 100644
--- a/.github/workflows/ci.yaml
+++ b/.github/workflows/ci.yaml
@@ -12,7 +12,7 @@ jobs:
       - uses: actions/checkout@v2
       - uses: ruby/setup-ruby@v1
         with:
-          ruby-version: '2.5.8'
+          ruby-version: '2.6.9'
           bundler-cache: true
       - name: Build documents
         run: |
diff --git a/Dockerfile b/Dockerfile
index 617f72e17..ca6548cef 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -1,4 +1,4 @@
-FROM ruby:2.5
+FROM ruby:2.6
 
 RUN gem install bundler jekyll
 
diff --git a/Gemfile.lock b/Gemfile.lock
index 53b98e302..31241ebb9 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -59,16 +59,16 @@ GEM
       rb-fsevent (~> 0.10, >= 0.10.3)
       rb-inotify (~> 0.9, >= 0.9.10)
     mercenary (0.3.6)
-    mini_portile2 (2.6.1)
+    mini_portile2 (2.8.0)
     minitest (5.14.1)
-    nokogiri (1.12.5)
-      mini_portile2 (~> 2.6.1)
+    nokogiri (1.13.3)
+      mini_portile2 (~> 2.8.0)
       racc (~> 1.4)
     parallel (1.12.1)
     pathutil (0.16.2)
       forwardable-extended (~> 2.6)
     public_suffix (4.0.6)
-    racc (1.5.2)
+    racc (1.6.0)
     rake (12.3.3)
     rb-fsevent (0.10.4)
     rb-inotify (0.10.1)