docker: make local builds work without version arg

[longbiaochen]

Fixes #7812

This makes the Dockerfile work for a plain local docker build . on Apple Silicon without requiring release-only build args.

Changes:

• give the Docker VERSION build arg a non-empty default (master) so ENV VERSION does not override Makefile version generation with an empty value
• add an optional MAKE_JOBS build arg while preserving the existing default nproc parallelism; low-memory local Docker VMs can pass --build-arg MAKE_JOBS=1

Verified on macOS Apple Silicon with Colima (linux/arm64 target, 8GB/4 CPU):

• docker build --target builder --progress=plain .
• docker build --progress=plain .

The original configurator failure no longer occurs on current master, but a plain Docker build currently fails later because empty VERSION prevents version_gen.h generation. With this patch the full default Docker image builds successfully.

[Copilot]

Pull request overview

Adjusts the root Dockerfile to make docker build . succeed in local environments (notably Apple Silicon) without requiring release-only build arguments, by ensuring a non-empty VERSION and limiting parallelism by default.

Changes:

• Set a non-empty default for Docker build arg VERSION (master) to avoid overriding Makefile version derivation with an empty value.
• Add MAKE_JOBS build arg (default 1) and use it for make -j... in the builder stage.

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[longbiaochen]

Updated after initial submission: MAKE_JOBS is now optional and defaults to the existing nproc behavior via ${MAKE_JOBS:-$(nproc)}. The only default behavior change is the non-empty VERSION=master build arg, which prevents an empty ENV VERSION from blocking version_gen.h generation during plain local Docker builds.

Re-verified after the update:

• docker build --target builder --progress=plain .
• docker build --progress=plain .

[longbiaochen]

Updated to address the Dockerfile review feedback. The builder no longer sets a global default VERSION/ENV; when VERSION is omitted, the Makefile derives the version from git, and VERSION is only passed into make when explicitly provided. MAKE_JOBS remains optional and falls back to nproc when omitted.\n\nRe-verified after this update:\n- docker build --target builder --progress=plain .\n- docker build --progress=plain .\n\nI used a temporary Docker config for the local build because this machine's Docker config references a missing docker-credential-desktop helper; the image build itself completed on the colima builder.