compose: Document that fig does not always rebuild images automatically

Fig does not seem to rebuild the image it uses to create a container when the referenced “build:” Dockerfile or directory has changed. More documentation about how using “build:” differs from “docker build” would be good.

E.g., I have directories “foo” and “foo-single”:

foo/Dockerfile

...

foo-single/Dockerfile

FROM foo
...

I changed foo/Dockerfile and rebuilt both images with “docker build -t foo ./foo” and “docker build -t foo-single ./foo-single” (though I guess that doesn’t matter, fig only looks at the build directory?).

My fig.yml has this so I expected it to pick up the new foo-single, but it does not. I even tried deleting the container (“docker rm myproject_bar”) thinking it would check for build changes, but it still used the outdated “myproject_bar” image .

bar:
  build: foo-single

I guess fig doesn’t check if the Dockerfile or its directory contents have changed? I had to explicitly “docker rmi myproject_bar” in order to get the new “foo” changes.