fix: expose env var to build (#3819)

Author: drdelambre

docker/Dockerfile.chronograf.prod

@@ -24,6 +24,8 @@ ARG UI_SHA
 ARG CLOUD_URL
 # Configure what honey badger uses for auth
 ARG HONEYBADGER_KEY
+# Configure the amplitude access key
+ARG AMPLITUDE_KEY
 # Throw any string up in the header, we use it for google tag manager
 ARG INJECT_HEADER
 # Injecting strings into the html never went wrong. we use this for google tag manager