devex: manage devtools in a project-local subdirectory + cleanup README (#829)

build/build.Dockerfile

@@ -0,0 +1,22 @@
+FROM !SUBSTITUTED_BY_MAKEFILE
+
+RUN apt-get update && apt-get install -y \
+    python3-pip \
+    python3-venv \
+    unzip \
+    gawk
+
+# setup venv for docs
+ENV VIRTUAL_ENV=/opt/venv
+RUN python3 -m venv $VIRTUAL_ENV
+ENV PATH="$VIRTUAL_ENV/bin:$PATH"
+ADD docs/requirements.txt /tmp/requirements.txt
+RUN pip3 install -r /tmp/requirements.txt
+
+# Go toolchain uses xdg-cache
+RUN mkdir -p /.cache && chmod -R 0777 /.cache
+
+# Go devtools are managed by Makefile
+
+WORKDIR /src
+