diff --git a/.devcontainer/Dockerfile b/.devcontainer/Dockerfile
new file mode 100644
index 0000000..e511a1b
--- /dev/null
+++ b/.devcontainer/Dockerfile
@@ -0,0 +1,7 @@
+FROM mcr.microsoft.com/devcontainers/jekyll
+
+# Fix: Remove the broken Yarn repository from the apt sources.
+# This prevents 'apt-get update' from failing due to the missing GPG key.
+# (Yarn is already provided by the dev container features/nvm, so this system repo is unnecessary)
+# See issue #3487
+RUN rm -f /etc/apt/sources.list.d/yarn.list
diff --git a/.devcontainer/devcontainer.json b/.devcontainer/devcontainer.json
index 46bd1ed..041b0e9 100644
--- a/.devcontainer/devcontainer.json
+++ b/.devcontainer/devcontainer.json
@@ -2,7 +2,10 @@
 // README at: https://github.com/devcontainers/templates/tree/main/src/jekyll
 {
   "name": "Jekyll",
-  "image": "mcr.microsoft.com/devcontainers/jekyll",
+  // Using a Dockerfile to fix the broken Yarn repository. See issue #3487.
+  "build": {
+    "dockerfile": "Dockerfile"
+  },
 
   // Features to add to the dev container. More info: https://containers.dev/features.
   "features": {