Would also vote for an extra button called "build" on the side of the "update" one ;-)

I am using dockerfile_inline and have the same problem (surprise :-)).

If there is no image configured, but a build section, update button could automatically build.
No need for an extra button, especially because update does nothing.
deploy should always build in this case.

On my side it's also related to watchtower.
As far as I read, watchtower uses the base image and does not see the changes in the compose file.
This seems to be ok, because I deploy anyways, if I change the compose file.
But if watchtower only downloads the image without applying the dockerfile, this would strip the additional parts from the image and I would need another solution instead of watchtower.
If at some point dockge already does all the basic work, it would be the ideal place to add some scheduling to do regular updates.

My 2 cents on the fact that the update button should also force build image if their were modifications within the build context is that maybe it will not be the behaviour expected but apart from that it seems perfectly OK IMO.

Adding some additional information. If you want update button to automatically re build the image. You can use pull_policy: build

Reference docker compose documentation

Not saying this solves the original ask of potential checking if the dockerfile was modified and forced the rebuild VS doing a rebuild every single time.

Sample:

services:
  my-dockerfile:
    build:
      context: .
      dockerfile: Dockerfile
  pull_policy: build
  restart: unless-stopped

thanks for the pointer, I didn't look into pull_policy because I didn't want to pull it :-)

this actually works for me (only the build, not the final goal):

...
  nodered:
    container_name: nodered
    #image: docker.io/nodered/node-red
    pull_policy: build
    build:
      context: .
      dockerfile_inline: |
        FROM docker.io/nodered/node-red:latest
        USER root
        RUN apk add --no-cache chromium
        USER node-red
    ...

the build takes longer for the first time and then runs quite fast (though it's always creating a new image, at least that's what it says).
If I change the dockerfile_inline value, then it takes longer again (e.g. the apk command if I add a package or if I add another RUN line).

And the inline file is shown in green, which is nice.