Update path used to activate venv.

Needs to be updated due to
zulip/zulip@d7556b4

Author: mateuszmandera

Dockerfile

@@ -39,7 +39,7 @@ ARG CUSTOM_CA_CERTIFICATES
 
 # Finally, we provision the development environment and build a release tarball
 RUN SKIP_VENV_SHELL_WARNING=1 ./tools/provision --build-release-tarball-only
-RUN . /srv/zulip-py3-venv/bin/activate && \
+RUN . ./.venv/bin/activate && \
     ./tools/build-release-tarball docker && \
     mv /tmp/tmp.*/zulip-server-docker.tar.gz /tmp/zulip-server-docker.tar.gz