build: enhance warning message when no output specified

[crazy-max]

fixes #968

fix the issue where no output warning message is being displayed for linked targets and also enhance this message to display which targets don't have an output specified.

# docker-bake.hcl

target "base" {
  dockerfile = "baseapp.Dockerfile"
  args = {
    basefoo = "bar"
  }
}

target "base2" {
  dockerfile = "baseapp.Dockerfile"
  args = {
    basefoo = "bar3"
  }
}

target "app" {
  contexts = {
    baseapp = "target:base"
  }
}

# Dockerfile
FROM baseapp
RUN echo "Hello world"

# baseapp.Dockerfile
FROM alpine
WORKDIR /src

$ docker buildx bake --set app.tags=user/app:latest --set app.output=type=cacheonly app
...
#5 [app 1/2] FROM docker.io/library/alpine@sha256:686d8c9dfa6f3ccfc8230bc3178d23f84eeaf7e457f36f271ab1acc53015037c
#5 resolve docker.io/library/alpine@sha256:686d8c9dfa6f3ccfc8230bc3178d23f84eeaf7e457f36f271ab1acc53015037c 0.0s done
#5 DONE 0.1s

#8 [base 2/2] WORKDIR /src
#8 CACHED

#9 [app 1/2] RUN echo "Hello world"
#9 CACHED

#8 [app 2/2] WORKDIR /src
#8 CACHED

$ docker buildx bake --set app.tags=user/app:latest --set app.output=type=cacheonly app base2
...
#7 [app 1/2] FROM docker.io/library/alpine@sha256:686d8c9dfa6f3ccfc8230bc3178d23f84eeaf7e457f36f271ab1acc53015037c
#7 resolve docker.io/library/alpine@sha256:686d8c9dfa6f3ccfc8230bc3178d23f84eeaf7e457f36f271ab1acc53015037c 0.0s done
#7 DONE 0.1s

#8 [app 2/2] WORKDIR /src
#8 CACHED
WARNING: No output specified for base2 target(s) with docker-container driver. Build result will only remain in the build cache. To push result image into registry use --push or to load image into docker use --load

$ docker buildx bake base base2
...
#6 [base2 1/2] FROM docker.io/library/alpine@sha256:686d8c9dfa6f3ccfc8230bc3178d23f84eeaf7e457f36f271ab1acc53015037c
#6 resolve docker.io/library/alpine@sha256:686d8c9dfa6f3ccfc8230bc3178d23f84eeaf7e457f36f271ab1acc53015037c 0.0s done
#6 DONE 0.0s

#7 [base 2/2] WORKDIR /src
#7 CACHED
WARNING: No output specified for base, base2 target(s) with docker-container driver. Build result will only remain in the build cache. To push result image into registry use --push or to load image into docker use --load

$ docker buildx build --build-context baseapp=docker-image://alpine .
WARNING: No output specified with docker-container driver. Build result will only remain in the build cache. To push result image into registry use --push or to load image into docker use --load
#1 [internal] load build definition from Dockerfile
#1 transferring dockerfile: 85B done
#1 DONE 0.0s

#2 [internal] load .dockerignore
#2 transferring context: 2B done
#2 DONE 0.1s
...

Signed-off-by: CrazyMax [email protected]

[tonistiigi]

Could the Linked property just be a helper function isLinked(targets, target) bool ?

[crazy-max]

@tonistiigi As discussed yesterday, we can't check current targets to find out if it's a linked one as it could also be explicitly specified by the user. See second example in #1148 (comment).

[tonistiigi]

Let's add a comment that Linked means the user did not request this target to be built.