Retool buildenv

[sam-at-luther]

Right now we supply build images which contain dockerfiles to create static images. But if we push dockerhub images for build + static then I think we could create multi-stage build in the scaffold repo that would optionally support a local GOPROXY for extra build speed .

[sam-at-luther]

Done — shipped as v0.1.0 (https://github.com/luthersystems/buildenv/releases/tag/v0.1.0).

What landed

Phase	Where
Vestigial multi-build-* matrix cleanup	#63
Substrate migration to multi-stage Dockerfile-go + GHA layer cache	luthersystems/substrate#307
Connectorhub migration	luthersystems/connectorhub#149
Common-infrastructure cleanup (no Go code left, dead-code deletion)	luthersystems/common-infrastructure#87
Lutherauth migration + bundled CVE bumps	luthersystems/lutherauth#28
Delete legacy Dockerfile.X.static + build-X.mk + DinD make static from buildenv	#64

Active downstream consumers (substrate, connectorhub, lutherauth, ui-core, cross-department-claims-settlement, sandbox-template family) are all on multi-stage FROM $BUILD_IMAGE Dockerfiles. Stale-pinned consumers (libmxf, metrics, datadog-monitor-aggregator) keep working against pre-v0.1.0 buildenv tags and can migrate at their own pace if they ever bump.

Not pursued

The "optional GOPROXY for extra build speed" half of the original issue is intentionally not implemented — build performance hasn't been the bottleneck in practice, and the GHA layer cache wired up in #62 already covers most of what it would have addressed. Easy to file a separate ticket if that ever changes.