Bind the app port deliberately and document the proxy deployment (closes #268) (closes #226)

Dockerfile

@@ -109,6 +109,18 @@ USER webhooker
 
 EXPOSE 8080
 
+# The binary defaults BIND_ADDRESS to 127.0.0.1, which is right for a
+# bare host: the cleartext listener serves the admin UI and the
+# unauthenticated receiver, so it must not appear on every interface
+# of a machine that configured nothing. A container is the other case.
+# Its network namespace is already the isolation boundary, so binding
+# every address inside it exposes nothing; what decides exposure is
+# the publish flag, and `-p 127.0.0.1:8080:8080` is the operator's
+# control there. Shipping the image on loopback would buy no security
+# and would make the process unreachable through its own published
+# port.
+ENV BIND_ADDRESS=0.0.0.0
+
 HEALTHCHECK --interval=30s --timeout=3s --start-period=5s --retries=3 \
     CMD wget --no-verbose --tries=1 --spider http://localhost:8080/.well-known/healthcheck || exit 1