server/docker: fix missing installation requirements

Furthermore, an update to Pillow has improved the floating-point
precision of the image hash algorithm, requiring minor updates to
the respective unit tests.

See https://github.com/python-pillow/Pillow/pull/4320

server/hooks/test

@@ -11,6 +11,7 @@ FROM ${BASE_IMAGE}
 WORKDIR /opt/app
 USER root
 RUN apk --no-cache add \
+		py3-pip \
 		py3-pytest \
 		py3-pytest-cov \
 		postgresql \