Allow specifying the image to use in Docker Compose

This is useful since the upcoming Terraform configuration is
going to pin down an image version for use on lnt.llvm.org.
Also, leave behind breadcrumbs to rebuild the image from
scratch, which is useful for local development.

Author: ldionne

docker/compose.yaml

@@ -12,13 +12,20 @@
 #   LNT_AUTH_TOKEN
 #     The authentication token used to require authentication to
 #     perform destructive actions.
+#
+# Additionally, the version of the llvm-lnt webserver image can be customized with
+# the LNT_IMAGE environment variable. It defaults to `latest`.
 
 name: llvm-lnt
 
 services:
   webserver:
     container_name: webserver
-    image: ghcr.io/llvm/llvm-lnt:latest
+    image: ghcr.io/llvm/llvm-lnt:${LNT_IMAGE:-latest}
+    # Useful for local debugging
+    # build:
+    #   context: ../
+    #   dockerfile: docker/lnt.dockerfile
     environment:
       - DB_USER=lntuser
       - DB_HOST=dbserver