Add --format option for --call outline and --call targets #3319
Description
Description
I was looking for a way to get the output of --call outline in a parseable format;
The default outputs both build-progress and the output (I'm still somewhat considering if that should be disabled by default).
Using --quiet helps for that;
docker build --call outline --quiet .
TARGET: binary
BUILD ARG VALUE DESCRIPTION
BASE_VARIANT alpine
ALPINE_VERSION 3.21 sets the version of the alpine base image to use, including for the golang image.
GO_VERSION 1.24.5
XX_VERSION 1.6.1
GOVERSIONINFO_VERSION v1.4.1
GO_LINKMODE static defines if static or dynamic binary should be produced
GO_BUILDTAGS defines additional build tags
GO_STRIP strips debugging symbols if set
CGO_ENABLED manually sets if cgo is used
VERSION sets the version for the produced binary
PACKAGER_NAME sets the company that produced the windows binaryTrying to use --format json (as is available on various other commands) failed;
docker build --call outline --quiet --format json .
unknown flag: --format
Usage: docker buildx build [OPTIONS] PATH | URL | -
Run 'docker buildx build --help' for more informationInstead, I tried to use the --progress=rawjson option (hoping it would be applied to both progress and output), but that failed;
docker build --call outline --quiet --progress rawjson .
ERROR: progress=rawjson and quiet cannot be used togetherRemoving the --quiet flag resulted in a mixture of json for progress and plain ("pretty printed") text for the output;
docker build --call outline --progress rawjson .
{"vertexes":[{"digest":"sha256:a6cbeff5225ceb525f3c621027662b713c1e9779d5254617275c898a12cb7e8f","name":"[internal] load build definition from Dockerfile"}]}
{"vertexes":
...
...
{"vertexes":[{"digest":"sha256:0b075d4c97dd3536b4530c234b91a25aa670565ad999e120c2b82096efb2114b","name":"[internal] load .dockerignore","started":"2025-07-16T08:37:47.238818877Z","completed":"2025-07-16T08:37:47.281984377Z"}]}
TARGET: binary
BUILD ARG VALUE DESCRIPTION
BASE_VARIANT alpine
ALPINE_VERSION 3.21 sets the version of the alpine base image to use, including for the golang image.
GO_VERSION 1.24.5
XX_VERSION 1.6.1
GOVERSIONINFO_VERSION v1.4.1
GO_LINKMODE static defines if static or dynamic binary should be produced
GO_BUILDTAGS defines additional build tags
GO_STRIP strips debugging symbols if set
CGO_ENABLED manually sets if cgo is used
VERSION sets the version for the produced binary
PACKAGER_NAME sets the company that produced the windows binaryIt would be nice to have an option to change the output format to produce a parseable JSON struct (which could be outlining the build-args per target and/or a list build args and targets.