Update Dockerfile mkdocs setup to fix build errors

Signed-off-by: Nikolai Thingnes Leira <nikolai.thingnes.leira@soprasteria.com>

.changeset/witty-buttons-admire.md

@@ -1,7 +0,0 @@
----
-'example-backend': patch
----
-
-Change base node image from node:18-bullseye-slim to node:18-bookworm-slim due to Docker build error on bullseye.
-
-You can apply these change to your own `Dockerfile` by replacing `node:18-bullseye-slim` with `node:18-bookworm-slim`

packages/backend/Dockerfile

@@ -18,9 +18,15 @@ FROM node:18-bookworm-slim
 RUN --mount=type=cache,target=/var/cache/apt,sharing=locked \
     --mount=type=cache,target=/var/lib/apt,sharing=locked \
     apt-get update && \
-    apt-get install -y --no-install-recommends libsqlite3-dev python3 python3-pip build-essential && \
-    yarn config set python /usr/bin/python3 && \
-    pip3 install mkdocs-techdocs-core==1.1.7
+    apt-get install -y --no-install-recommends libsqlite3-dev python3 python3-pip python3-venv build-essential && \
+    yarn config set python /usr/bin/python3
+
+# Set up a virtual environment for mkdocs-techdocs-core.
+ENV VIRTUAL_ENV=/opt/venv
+RUN python3 -m venv $VIRTUAL_ENV
+ENV PATH="$VIRTUAL_ENV/bin:$PATH"
+
+RUN pip3 install mkdocs-techdocs-core==1.1.7
 
 # From here on we use the least-privileged `node` user to run the backend.
 WORKDIR /app