Fix installation of custom plugin

templates/api/Dockerfile

@@ -3,8 +3,11 @@ FROM alerta/alerta-web
 USER root
 COPY ./plugins/*.py /tmp/plugins/
 RUN /venv/bin/pip install python-gitlab && \
-    /venv/bin/python3 /tmp/plugins/setup_alerta_gitlab.py install && \
-    /venv/bin/python3 /tmp/plugins/setup_alerta_elastalert.py install && \
+    cd /tmp/plugins && \
+    /venv/bin/python3 setup_alerta_gitlab.py install && \
+    /venv/bin/python3 setup_alerta_elastalert.py install && \
+    cp alerta_gitlab.py /venv/lib/python3.7/site-packages/ && \
+    cp alerta_elastalert.py /venv/lib/python3.7/site-packages/ && \
     echo "Done"
 
 USER 1001