Tab-completion for `docker build` is partially broken

Description

• relates to https://github.com/docker/cli/pull/5516

When trying tab-completion for docker build, it looks like it's trying to complete subcommands (this should probably complete to file-paths / directory names);

docker build<TAB>
build    builder  buildx

Trying to provide completion for a flag that has completion set locally does complete, but does not use buildx's completion;

docker build --platform<TAB>
linux           linux/amd64     linux/arm/v5    linux/arm/v7    linux/arm64/v8  linux/riscv64   wasip1          windows
linux/386       linux/arm       linux/arm/v6    linux/arm64     linux/ppc64le   linux/s390x     wasip1/wasm     windows/amd64

Completion for docker buildx build uses local files (which would be expected for the build-context), and forcing it to complete options (--<tab>) works;

docker buildx build --<TAB>
--add-host         (Add a custom host-to-IP mapping (format: "host:ip"))
--allow            (Allow extra privileged entitlement (e.g., "network.host", "security.insecure"))
--annotation       (Add annotation to the image)
--attest           (Attestation parameters (format: "type=sbom,generator=image"))

Reproduce the completion cases for docker build and docker buildx build, then inspect the completion implementation and related pull request #5516. Compare subcommand, build-context, flag, and flag-value completion; done means docker build completes paths or directories and its flags and values behave consistently with buildx build.