docker: allow setting Ames port via docker run argument

nix/pkgs/docker-image/default.nix

@@ -5,6 +5,20 @@ let
 
     set -eu
 
+    # set defaults
+    amesPort=${toString amesPort}
+
+    # check args
+    for i in "$@"
+    do
+    case $i in
+        -p=*|--port=*)
+            amesPort="${i#*=}"
+            shift
+            ;;
+    esac
+    done
+
     # If the container is not started with the `-i` flag
     # then STDIN will be closed and we need to start
     # Urbit/vere with the `-t` flag.
@@ -23,7 +37,7 @@ let
       mv $keyname /tmp
 
       # Boot urbit with the key, exit when done booting
-      urbit $ttyflag -w $(basename $keyname .key) -k /tmp/$keyname -c $(basename $keyname .key) -p ${toString amesPort} -x
+      urbit $ttyflag -w $(basename $keyname .key) -k /tmp/$keyname -c $(basename $keyname .key) -p $amesPort -x
 
       # Remove the keyfile for security
       rm /tmp/$keyname
@@ -34,7 +48,7 @@ let
       cometname=''${comets[0]}
       rm *.comet
 
-      urbit $ttyflag -c $(basename $cometname .comet) -p ${toString amesPort} -x
+      urbit $ttyflag -c $(basename $cometname .comet) -p $amesPort -x
     fi
 
     # Find the first directory and start urbit with the ship therein
@@ -42,7 +56,7 @@ let
     dirs=( $dirnames )
     dirname=''${dirnames[0]}
 
-    exec urbit $ttyflag -p ${toString amesPort} $dirname
+    exec urbit $ttyflag -p $amesPort $dirname
     '';