Include some dockerfile changes from develop

I think these may have been retained in merge confusion.

docker/Dockerfile

@@ -107,7 +107,7 @@ RUN pip install --prefix="/install" --no-deps --no-warn-script-location /synapse
 ### Stage 2: runtime
 ###
 
-FROM base
+FROM docker.io/python:${PYTHON_VERSION}-slim
 
 LABEL org.opencontainers.image.url='https://matrix.org/docs/projects/server/synapse'
 LABEL org.opencontainers.image.documentation='https://github.com/matrix-org/synapse/blob/master/docker/README.md'
@@ -129,7 +129,7 @@ RUN \
     openssl \
     && rm -rf /var/lib/apt/lists/*
 
-COPY --from=builder /synapse/ /synapse
+COPY --from=builder /install /usr/local
 COPY ./docker/start.py /start.py
 COPY ./docker/conf /conf
 

docker/start.py

@@ -1,4 +1,4 @@
-#!/synapse/.venv/bin/python
+#!/usr/local/bin/python
 
 import codecs
 import glob