Add trustedproxy to the proxy image build context

724c6a0 introduced the top-level trustedproxy package and the proxy
imports it, but proxy/Dockerfile.multistage enumerates copied
directories and was not updated, so the agent-network e2e harness
failed to build the proxy image (module provides no such package).
The combined and client images COPY the whole tree and are unaffected.

proxy/Dockerfile.multistage

@@ -14,6 +14,7 @@ COPY proxy ./proxy
 COPY route ./route
 COPY shared ./shared
 COPY sharedsock ./sharedsock
+COPY trustedproxy ./trustedproxy
 COPY upload-server ./upload-server
 COPY util ./util
 COPY version ./version