fix: use pip3 and python3 -m mkdocs for Drone compatibility

The Drone runner environment does not have `pip` or `mkdocs` on PATH
even when installed via the Docker image. Use `pip3` explicitly and
invoke mkdocs as `python3 -m mkdocs` to ensure reliable execution.
Also add --break-system-packages to the fallback install.

Dockerfile

@@ -9,7 +9,7 @@ RUN apt-get update && apt-get install -y \
 	python3 python3-pip && \
 	apt-get clean my room
 
-RUN pip install --no-cache-dir --break-system-packages "mkdocs>=1.6,<2" mkdocs-material
+RUN pip3 install --no-cache-dir --break-system-packages "mkdocs>=1.6,<2" mkdocs-material
 
 SHELL ["/bin/bash", "-c"]
 ARG FPC_VERSION=3.2.2