feat(build): publish multi-arch container image to GHCR on release

[stevapple]

Closes #630

Publishes modctl as a multi-arch container image so it can be used in CI/CD without installing the Go toolchain.

Changes

• build/Dockerfile — cross-compiling builder (golang:1.25 on $BUILDPLATFORM) plus a gcr.io/distroless/static-debian13:nonroot runtime, matching the layout used by model-csi-driver. The version ldflags mirror the ones in the existing release build, so modctl version reports the same fields from the image as from the released tarballs.
• .github/workflows/release.yaml — a publish-image job that pushes ghcr.io/modelpack/modctl:<tag> and :latest for linux/amd64 and linux/arm64 on every v* tag, using GITHUB_TOKEN with packages: write.

The job is independent of the existing binary matrix, and because the builder cross-compiles it needs no QEMU — the whole job takes about as long as a single go build.

Why CGO_ENABLED=0

The released Linux tarball binaries are built with CGO_ENABLED=1 and a statically linked glibc, which breaks NSS-backed os/user lookups. Dropping such a binary into a distroless base panics before any command runs:

$ docker run --rm modctl:repackaged version
panic: user: unknown userid 65532

goroutine 1 [running]:
github.com/modelpack/modctl/cmd.init.12()
	cmd/root.go:117 +0x444

config.NewRoot() calls user.Current() to derive the default storage and log directories, and panics if it fails. This is the same class of failure as #285 / #433 (getgrgid_r segfault in the static binaries).

Building with CGO_ENABLED=0 avoids it entirely: os/user uses the pure-Go implementation that reads /etc/passwd (where distroless does have an entry for uid 65532), and the build selects the pure-Go go-git backend via the existing //go:build !enable_libgit2 constraint in pkg/source/git_gogit.go.

Worth flagging for reviewers: this means the image binary uses go-git while the tarball binaries use libgit2. model-csi-driver already builds its release binaries with CGO_ENABLED=0, so there is precedent in the org, but if you would rather the image match the tarballs exactly, the alternative is to keep CGO and add the osusergo/netgo tags proposed in #433 — happy to switch.

Testing

Built locally with docker buildx for both platforms:

$ docker buildx build --platform linux/amd64,linux/arm64 \
    --build-arg GITVERSION=v0.0.0-poc --build-arg GITCOMMIT=bd2c9f7 \
    -f build/Dockerfile -t modctl:multi .

The resulting manifest list carries linux/amd64 and linux/arm64 (42–46 MB per architecture). Both run as the non-root user:

$ docker run --rm modctl:arm64 version
Version:    v0.0.0-poc
Commit:     bd2c9f7
Platform:   linux
BuildTime:  2026-08-12T14:16:45Z

End-to-end check against Qwen2.5-0.5B-Instruct mounted into the container:

$ docker run --rm -v ./qwen2.5-0.5b-instruct:/model:ro modctl:arm64 \
    modelfile generate /model --name qwen2.5-0.5b-instruct \
    --format safetensors --param-size 0.5b -O /tmp
Generating modelfile for /model
Successfully generated modelfile:
NAME qwen2.5-0.5b-instruct
ARCH transformer
FAMILY qwen2
FORMAT safetensors
PARAMSIZE 0.5b
PRECISION bfloat16
CONFIG config.json
...
MODEL model.safetensors
DOC LICENSE
DOC README.md

Notes

• The image has no shell, so docker run ... modctl <args> works but sh -c style invocations do not. If you would prefer a shell for CI systems that inject commands: blocks, ubuntu:24.04 (as in model-csi-driver) is a drop-in change to the final stage.