fix: invalid nested interpolation (#1496)

Nested interpolation is not supported by docker (docker/cli#4265). Changed the file so that it uses `CUSTOM_TAG` or `ERPNEXT_VERSION` if that's not available.

compose.yaml

@@ -2,7 +2,7 @@ x-customizable-image: &customizable_image
   # By default the image used only contains the `frappe` and `erpnext` apps.
   # See https://github.com/frappe/frappe_docker/blob/main/docs/custom-apps.md
   # about using custom images.
-  image: ${CUSTOM_IMAGE:-frappe/erpnext}:${CUSTOM_TAG:-${ERPNEXT_VERSION:?No ERPNext version or tag set}}
+  image: ${CUSTOM_IMAGE:-frappe/erpnext}:${CUSTOM_TAG:-$ERPNEXT_VERSION}
   pull_policy: ${PULL_POLICY:-always}
 
 x-depends-on-configurator: &depends_on_configurator