add requirements.txt instead of pip install dash[diskcache]

I initially tried adding dash[diskcache] to the requirements.txt but couldn't make it work. It turns out that all `pip install dash[diskcache]` does is include these 3 packages which are optional dependencies of dash
https://github.com/plotly/dash/blob/dev/requirements/diskcache.txt
Adding these to the requirements.txt has the same effect.

docker/setup.sh

@@ -1,4 +1,3 @@
 #!/bin/bash
 source setup/activate.sh 
 pip install -r dashboard_setup/requirements.txt
-pip install dash[diskcache]

requirements.txt

@@ -16,3 +16,8 @@ flask-talisman==1.0.0
 dash_auth==2.0.0
 arrow==1.3.0
 dash-leaflet==1.0.7
+# dash[diskcache] dependencies
+# from https://github.com/plotly/dash/blob/dev/requirements/diskcache.txt
+diskcache>=5.2.1
+multiprocess>=0.70.12
+psutil>=5.8.0