build requirements + system packages before copying app code

Dockerfile

@@ -3,7 +3,7 @@ ARG BRANCH_NAME=master
 ENV BRANCH_NAME ${BRANCH_NAME}
 ENV TINI_VERSION v0.19.0
 ENV PMM_DOCKER True
-COPY . /
+COPY requirements.txt requirements.txt
 RUN echo "**** install system packages ****" \
  && apt-get update \
  && apt-get upgrade -y --no-install-recommends \
@@ -19,5 +19,6 @@ RUN echo "**** install system packages ****" \
  && apt-get -f install \
  && apt-get autoclean \
  && rm -rf /requirements.txt /tmp/* /var/tmp/* /var/lib/apt/lists/*
+ COPY . /
 VOLUME /config
 ENTRYPOINT ["/tini", "-s", "python3", "plex_meta_manager.py", "--"]