docker: install required Python packages to build documentation

Otherwise, `make documentation` fails in the devcontainer.

doc/requirements.txt

 breathe
+sphinx
 sphinx-rtd-theme

docker/Dockerfile.buildenv

@@ -30,8 +30,6 @@ RUN apt-get update \
 	python-is-python3 \
 	python3-dev \
 	python3-pip \
-	python3-sphinx \
-	python3-sphinx-rtd-theme \
 	rsync \
 	scons \
 	unzip \
@@ -40,7 +38,7 @@ RUN apt-get update \
 	vim \
  && rm -rf /var/lib/apt/lists/*
 COPY requirements.txt /tmp/requirements.txt
-RUN pip install -r /tmp/requirements.txt
+RUN pip install --no-cache-dir --upgrade -r /tmp/requirements.txt
 COPY verilator.patch /tmp/
 RUN cd /tmp \
  && git clone -b v4.010 https://github.com/verilator/verilator \

docker/rules.mk

@@ -33,7 +33,7 @@ DOCKER_IMAGES := simbricks/simbricks-build simbricks/simbricks-base \
 REQUIREMENTS_TXT := $(d)requirements.txt
 
 $(REQUIREMENTS_TXT):
-	cp requirements.txt $@
+	cat requirements.txt doc/requirements.txt > $@
 
 docker-images: $(REQUIREMENTS_TXT)
 	docker build -t \