incorrect "ERROR: tag is needed when pushing to registry" when name is missing

[KonradHoeffner]

Contributing guidelines

• I've read the contributing guidelines and wholeheartedly agree

I've found a bug and checked that ...

• ... the documentation does not mention anything about my problem
• ... there are no open or closed issues that are related to my problem

Description

When using buildx with a tag but without a name, it incorrectly reports that a tag is needed.
This can lead to a lot of wasted time when debugging for example GitHub actions where environment variables may be used and one of them is set incorrectly.
The user now expects the environment variable for the tag being empty when in fact it is the name that is missing.

Expected behaviour

docker buildx build . --output type=image,push-by-digest=true,push=true,tag=123 should return:

ERROR: name is needed when pushing to registry

Actual behaviour

docker buildx build . --output type=image,push-by-digest=true,push=true,tag=123 results in:

ERROR: tag is needed when pushing to registry

Buildx version

0.20.1

Docker info

Builders list

NAME/NODE     DRIVER/ENDPOINT   STATUS    BUILDKIT   PLATFORMS
default*      docker                                 
 \_ default    \_ default       running   v0.18.2    linux/amd64 (+3), linux/arm64, linux/arm (+2), linux/ppc64le, (7 more)

Configuration

No Dockerfile needed, this happens even when calling it in a directory without a Dockerfile.

Build logs

Additional info

This seems to be caused by

buildx/build/build.go

Line 360 in 2389d45

return errors.Errorf("tag is needed when pushing to registry")

and also in

buildx/build/build.go

Line 547 in 2389d45

return errors.Errorf("tag is needed when pushing to registry")

:

pushNames = e.Attrs["name"]
if pushNames == "" {
	return errors.Errorf("tag is needed when pushing to registry")
}

[tonistiigi]

--tag is the flag of the build command.