docker: Upgrade pip before installing requirements

Outdated pip versions require a rust compiler to install the
cryptography package. Ensuring that pip is up to date should eliminate
the recent buildx errors where a prebuilt cryptography wheel is not
available.

Dockerfile

@@ -9,6 +9,7 @@ RUN apk --update add \
 
 COPY requirements.txt .
 
+RUN pip install --upgrade pip
 RUN pip install --prefix /install --no-warn-script-location --no-cache-dir -r requirements.txt
 
 FROM python:3.8-alpine