Makefile: use linuxkit env vars for org and builder config

[rucoder]

Description

NOTES:

1. LK is not released yet
2. please review as if all LK commits are merged. this is IMO the way to drive CI
3. sure the build fails since 1.8.3 (or whatever next version is) is not yet released

linuxkit v1.8.3 introduces native environment variable support for flags previously threaded through every linuxkit call site in this Makefile via LINUXKIT_ORG_TARGET and BUILDKIT_CONFIG_OPTS. This PR removes those Makefile-level workarounds and lets linuxkit pick up configuration directly from the runner environment.

Removed:

• BUILDKIT_CONFIG_FILE / BUILDKIT_CONFIG_OPTS variables and the manifest-mode guard
• LINUXKIT_ORG_TARGET variable and its injection at 9 call sites
• REGISTRY make variable (was only used to feed LINUXKIT_ORG_TARGET)

New interface — set in the runner environment or on the make command line:

Purpose	Old	New
Custom push registry	make REGISTRY=myregistry	LINUXKIT_PKG_ORG=myregistry/lfedge make
buildkit registry mirrors	BUILDKIT_CONFIG_FILE=/etc/buildkit/buildkitd.toml (auto-detected)	LINUXKIT_BUILDER_CONFIG=/path/to/buildkitd.toml (explicit)
Pull-side registry mirrors	not supported	LINUXKIT_MIRROR=docker.io=http://my-mirror.local

A $(warning ...) is emitted if /etc/buildkit/buildkitd.toml exists on the local machine but LINUXKIT_BUILDER_CONFIG is not set, to avoid silently losing a previously auto-detected config.

Bumps LINUXKIT_VERSION to v1.8.3.

PR dependencies

Requires linuxkit v1.8.3 which includes:
linuxkit/linuxkit#4205

How to test and validate this PR

1. Normal build — make without any overrides should behave identically to before.
2. Custom registry — LINUXKIT_PKG_ORG=myregistry/lfedge make should tag/push packages to myregistry/lfedge/* (replaces make REGISTRY=myregistry).
3. buildkit mirror — set LINUXKIT_BUILDER_CONFIG=/etc/buildkit/buildkitd.toml on a runner that has the file; verify buildkit uses the mirror config.
4. Warning — create /etc/buildkit/buildkitd.toml locally without setting LINUXKIT_BUILDER_CONFIG; confirm the warning is printed.

Changelog notes

No user-facing changes. Build tooling simplification: linuxkit registry mirror and org configuration is now passed via environment variables (LINUXKIT_BUILDER_CONFIG, LINUXKIT_PKG_ORG, LINUXKIT_MIRROR) instead of make variables, making CI runner configuration more transparent.

PR Backports

• 16.0-stable: No
• 14.5-stable: No.
• 13.4-stable: No.

Checklist

• I've provided a proper description
• I've added the proper documentation
• I've tested my PR on amd64 device
• I've tested my PR on arm64 device
• I've written the test verification instructions
• I've set the proper labels to this PR
• I've checked the boxes above, or I've provided a good reason why I didn't check them.- [ ]

[rene]

I personally like the REGISTRY variable, it's intuitive and easy to use (no need to provide the org). But if you are insist to remove it, then the documentation must be updated: https://github.com/lf-edge/eve/blob/master/docs/BUILD.md#using-a-local-container-registry

[rucoder]

@rene i know you love it but I just wanted to make the experience consistent , I do not insist, we can keep it if you want but IMO it maybe introduce more confusion in the future

[rene]

ok, so update the documentation....

[europaul]

LGTM, I only wish LINUXKIT_BUILDER_CONFIG would be called BUILDKIT_CONFIG_PATH but I think it's too late for that :)

[rucoder]

LGTM, I only wish LINUXKIT_BUILDER_CONFIG would be called BUILDKIT_CONFIG_PATH but I think it's too late for that :)

@europaul no, not to late. but it must be exactly BUILDKIT_CONFIG_PATH or LINUXKIT_BUILDER_CONFIG_PATH would work too? or BUILDKIT_CONFIG_PATH is some pre-defined buildkit variable?

[europaul]

LGTM, I only wish LINUXKIT_BUILDER_CONFIG would be called BUILDKIT_CONFIG_PATH but I think it's too late for that :)

@europaul no, not to late. but it must be exactly BUILDKIT_CONFIG_PATH or LINUXKIT_BUILDER_CONFIG_PATH would work too? or BUILDKIT_CONFIG_PATH is some pre-defined buildkit variable?

It's just an arbitrary name but I think buildkit should be included instead of builder since it provides clarity as to what this config actually is. builder is too abstract