chore: add Docker image for docs and Uncloud Compose deployment (dogfooding!)

docs/Dockerfile

@@ -0,0 +1,27 @@
+# Stage 1: Base image.
+## Start with a base image containing NodeJS so we can build Docusaurus.
+FROM node:lts-alpine AS base
+## Disable colour output from yarn to make logs easier to read.
+ENV FORCE_COLOR=0
+## Enable corepack.
+RUN corepack enable
+## Set the working directory to `/opt/docusaurus`.
+WORKDIR /opt/docusaurus
+
+# Stage 2: Production build.
+FROM base AS prod
+## Set the working directory to `/opt/docusaurus`.
+WORKDIR /opt/docusaurus
+## Copy over the source code.
+COPY . /opt/docusaurus/
+## Install dependencies with `--immutable` to ensure reproducibility.
+RUN npm ci
+## Build the static site.
+RUN npm run build
+
+# Stage 3: Serve static site with Caddy.
+FROM caddy:2.10.0-alpine AS caddy
+## Copy the Caddyfile.
+COPY ./Caddyfile /etc/caddy/Caddyfile
+## Copy the Docusaurus build output.
+COPY --from=prod /opt/docusaurus/build /usr/share/caddy