feat: minimize gateway Docker image to reduce attack surface and pull size

Problem Statement

The gateway runtime image is based on nvcr.io/nvidia/base/ubuntu:noble-20251013, a full Ubuntu userspace. While the multi-stage build already avoids shipping build tools, the final image still carries unnecessary OS packages, shells, and utilities that increase:

1. Attack surface — a compromised container has access to package managers, shells, and system utilities that aid lateral movement.
2. Image size — larger images slow cold-start pulls, especially in air-gapped or bandwidth-constrained environments.
3. CVE noise — distro packages accumulate vulnerabilities unrelated to the gateway binary, increasing scanner findings and maintenance burden.

Proposed Design

1. Switch base image to distroless

Replace nvcr.io/nvidia/base/ubuntu:noble with gcr.io/distroless/cc-debian13:nonroot.

This image includes glibc and libgcc — sufficient for the dynamically-linked Rust binary (built with x86_64-unknown-linux-gnu / aarch64-unknown-linux-gnu targets) — but ships no shell, no package manager, and no unnecessary utilities. It is ~20 MB compressed vs ~80 MB for the Ubuntu base.

A follow-up issue can evaluate switching to static-debian13 with a musl-linked binary for an even smaller footprint (~2 MB base), but that requires target/toolchain changes and broader testing.

2. Remove unnecessary migrations COPY

The current Dockerfile copies crates/openshell-server/migrations/ into the final image. This is unnecessary — migrations are embedded into the binary at compile time via sqlx::migrate!("./migrations/sqlite") and sqlx::migrate!("./migrations/postgres") proc macros. Removing this COPY eliminates dead weight from the image.

3. Minimal final stage

FROM gcr.io/distroless/cc-debian13:nonroot AS gateway
COPY --from=gateway-binary /build/out/openshell-gateway /usr/local/bin/
ENTRYPOINT ["openshell-gateway"]
CMD ["--port", "8080"]

Optionally the openshell user (UID 1000) can be preserved.

CA certs are included in the distroless base. No other runtime files are needed.

Alternatives Considered

• distroless/static-debian13 — smallest possible (~2 MB) but requires switching to x86_64-unknown-linux-musl and verifying all crate dependencies work under musl. Better as a follow-up.
• Alpine-based image — smaller than Ubuntu but still ships a shell, package manager, and introduces musl quirks (DNS resolver, stack size defaults).
• Keep Ubuntu, remove packages — fragile (apt remove can break dep chains), doesn't eliminate shell or package manager, still large.
• Chainguard cgr.dev/chainguard/cc-dynamic — signed and SBOM-attached but adds an external registry dependency.

Agent Investigation

Current state of deploy/docker/Dockerfile.images gateway stage:

• Multi-stage build already separates builder from runtime.
• Final stage: FROM nvcr.io/nvidia/base/ubuntu:noble-20251013 AS gateway
• Installs ca-certificates and upgrades gpgv at runtime.
• Copies binary + migrations (migrations COPY is unnecessary — embedded via sqlx::migrate!()).
• Runs as openshell user (UID 1000).
• Debug symbols already stripped (strip = true in [profile.release]).
• Binary is dynamically linked against glibc (*-unknown-linux-gnu targets in cross-build.sh).

Estimated compressed image size: current ~100+ MB → ~30-40 MB with cc-debian13 + removing dead COPY.

Checklist

• I've reviewed existing issues and the architecture docs
• This is a design proposal, not a "please build this" request