Allow exposed metrics port to be overridden

Rekor uses the same metrics port, so if both services are run with
docker-compose they conflict with each other. Add an environment
variable to expose the metrics service on a different docker port.

Signed-off-by: Colleen Murphy <colleenmurphy@google.com>

docker-compose.yml

@@ -34,7 +34,7 @@ services:
     ports:
       - "5555:5555"
       - "5554:5554"
-      - "2112:2112"
+      - "${FULCIO_METRICS_PORT:-2112}:2112"
     volumes:
       - ~/.config/gcloud:/root/.config/gcloud/:z # for GCP authentication
       - ./config/config.jsn:/etc/fulcio-config/config.json:z

docs/setup.md

@@ -12,6 +12,14 @@ Simply run:
 docker-compose up
 ```
 
+If you need to change the docker port on which the metrics port is bound, use
+the `FULCIO_METRICS_PORT` variable:
+
+```
+export FULCIO_METRICS_PORT=2113
+docker-compose up
+```
+
 The other way is running the Fulcio binary:
 
 ```