Enable caching for proxied content

[jon-betts]

To test this:

• make docker
• docker stop via3_nginx-proxy_1; docker rm via3_nginx-proxy_1; make services
• Now you should be able to follow the following URLs with the following outcomes:
• curl -I http://localhost:9083/proxy/static/http://pdf995.com/samples/pdf.pdf
  • Upstream has Cache-Control
  • Cache-Control: public, max-age=86400, stale-while-revalidate=604800
• curl -I http://localhost:9083/proxy/static/http://africau.edu/images/default/sample.pdf
  • Upstream has no Cache-Control
  • Cache-Control: public, max-age=86400, stale-while-revalidate=604800
• curl -I http://localhost:9083/proxy/static/http://httpbin.org/status/500
  • No cache header
• curl -I http://localhost:9083/proxy/static/http://httpbin.org/status/500
  • No cache header

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Net effect:

• Previously Cloudflare defaults kicked in (now 30m)
• Now we set to 1 day
• We now set stale while revalidate