diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index c82bf650..25fb1457 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -21,7 +21,7 @@ jobs:
 
     steps:
       - name: Checkout
-        uses: actions/checkout@v6.0.2
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
         with:
           fetch-depth: 10
 
@@ -30,12 +30,12 @@ jobs:
           ruby-version: ${{ matrix.ruby }}
           bundler-cache: true
 
-      - uses: actions/setup-python@v6.2.0
+      - uses: actions/setup-python@a309ff8b426b58ec0e2a45f0f869d46889d02405 # v6.2.0
         with:
-          # This should match lib/github/markups.rb GitHub::Markups::MARKUP_RST
+          # Required by lib/github/commands/rest2html (RST rendering)
           python-version: "3.x"
 
-      - uses: actions/cache@v5.0.4
+      - uses: actions/cache@668228422ae6a00e4ad889ee87cd7109ec5666a7 # v5.0.4
         with:
           path: ~/.cache/pip
           key: ${{ runner.os }}-pip
@@ -52,7 +52,9 @@ jobs:
           sudo cpanm --installdeps --notest Pod::Simple
 
       - name: Install Python dependencies
-        run: python -m pip install docutils
+        run: |
+          echo 'docutils==0.22.4 --hash=sha256:d0013f540772d1420576855455d050a2180186c91c15779301ac2ccb3eeb68de' > /tmp/requirements.txt
+          python -m pip install -r /tmp/requirements.txt
 
       - name: Run rake
         run: |
diff --git a/Dockerfile b/Dockerfile
index aa0f09aa..eade13c8 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -1,4 +1,4 @@
-FROM ubuntu:trusty
+FROM ubuntu:trusty@sha256:64483f3496c1373bfd55348e88694d1c4d0c9b660dee6bfef5e12f43b9933b30 # trusty
 
 RUN apt-get update -qq
 RUN apt-get install -y apt-transport-https
@@ -18,7 +18,7 @@ RUN install-zef-as-user && zef install Pod::To::HTML
 RUN curl -L http://cpanmin.us | perl - App::cpanminus
 RUN cpanm --installdeps --notest Pod::Simple
 
-RUN pip install docutils
+RUN pip install docutils==0.18.1
 
 ENV PATH $PATH:/root/.rbenv/bin:/root/.rbenv/shims
 RUN curl -fsSL https://github.com/rbenv/rbenv-installer/raw/master/bin/rbenv-installer | bash
@@ -26,7 +26,7 @@ RUN rbenv install 2.4.1
 RUN rbenv global 2.4.1
 RUN rbenv rehash
 
-RUN gem install bundler
+RUN gem install bundler -v 2.3.26
 
 WORKDIR /data/github-markup
 COPY github-markup.gemspec .
diff --git a/Gemfile.lock b/Gemfile.lock
index 23b61036..525e2c94 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -138,4 +138,4 @@ DEPENDENCIES
   wikicloth (= 0.8.3)
 
 BUNDLED WITH
-   2.4.10
+   2.3.26