Use a virtual env for pip requirements, otherwise we get conflicts now

(due to pinning all requirements with pip-tools)

Dockerfile

@@ -94,10 +94,16 @@ WORKDIR /usr/src/app
 COPY requirements.txt /usr/src/app/
 COPY requirements_dev.txt /usr/src/app/
 COPY entrypoint.sh /usr/src/app/
-ENV PATH=$HOME/.cargo/bin:$PATH
+ENV VIRTUAL_ENV=/usr/src/venv
+ENV PATH=$VIRTUAL_ENV/bin:$HOME/.cargo/bin:$PATH
 
 RUN apk -U upgrade
+# Use a virtual env here, because othewise we get conflicats between Alpine's
+# packages and pip's. (This has started happening because we switched to pip-
+# tools which pins every dependency).
 RUN apk add --no-cache gcc musl-dev python3-dev libffi-dev openssl-dev cargo libwebp libwebp-tools &&\
+        python3 -m venv /usr/src/venv &&\
+        . /usr/src/venv/bin/activate &&\
         pip3 install -r requirements_dev.txt
 
 RUN apk add --no-cache gettext