Add minimal dev container configuration

For editing the site using a dev container locally, or in Codespaces
online.

The `deadsnakes` configuration is taken from the
`research-template-docker` repository:

https://github.com/opensafely-core/research-template-docker/blob/669a76d79c198925d788dedc34fe3c5a831872b5/Dockerfile#L20-L21

which avoids installing `software-properties-common` first (to get the
`add-apt-repository` command.

The dev container settings are based on the Bennett site configuration,
with a longer autosave delay, because the site currently takes a while
to build (#1397). Arguably, the save delay could be longer still.

No Markdown plugins are installed yet; maybe that's something we could
do in future.

.devcontainer/Dockerfile

@@ -0,0 +1,16 @@
+FROM mcr.microsoft.com/devcontainers/base:ubuntu-24.04
+
+RUN echo "deb http://ppa.launchpadcontent.net/deadsnakes/ppa/ubuntu noble main" > /etc/apt/sources.list.d/deadsnakes-ppa.list && \
+    /usr/lib/apt/apt-helper download-file 'https://keyserver.ubuntu.com/pks/lookup?op=get&search=0xf23c5a6cf475977595c89f51ba6932366a755776' /etc/apt/trusted.gpg.d/deadsnakes.asc
+
+RUN apt-get update && \
+    apt-get install -y \
+    just \
+    python3.11 \
+    python3.11-venv
+
+USER vscode
+
+# Clone cohort-extractor submodule via HTTPS.
+# GitHub does not allow SSH access without adding a private key.
+RUN git config --global url.https://github.com/.insteadOf [email protected]:

.devcontainer/devcontainer.json

@@ -0,0 +1,15 @@
+{
+        "name": "Documentation development",
+        "dockerFile": "Dockerfile",
+        "postAttachCommand": "just run",
+        "customizations": {
+                "vscode": {
+                        "settings": {
+                                "extensions.ignoreRecommendations": true,
+                                "files.autoSave": "afterDelay",
+                                "files.autoSaveDelay": 3000,
+                                "git.autofetch": true
+                        }
+                }
+        }
+}