fix(docker): build the musl/Alpine image lean (--no-default-features) (#242)

The Docker image build has been broken since the bundled-embeddings default
(#237/#238): on Alpine/musl, `cargo build --release` now pulls in `ort`
(ONNX Runtime), whose prebuilt binaries are glibc-only, and the download chain
needs `openssl-sys` — which fails on Alpine (`Package 'openssl' not found`).
musl + ONNX Runtime is fundamentally incompatible.

This Dockerfile is explicitly the static-musl binary for the Firecracker
microVM sandbox, so the lean build is the correct artifact here: build with
`--no-default-features` (no bundled embeddings), restoring the pre-#238 feature
set. FTS5 keyword recall works out of the box; dense/hybrid can use an external
embedder. A semantic-search-by-default image would need a glibc base (follow-up).

Also COPY build.rs (added in #238; Cargo now requires it to be present even when
the model-fetch path is gated off under --no-default-features).

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: tcconnally

Dockerfile

@@ -1,11 +1,20 @@
 # Glama-compatible Dockerfile for Mimir
-# Builds a static musl binary for Firecracker microVM sandbox execution
+# Builds a static musl binary for Firecracker microVM sandbox execution.
+#
+# This is the LEAN build (--no-default-features): no bundled ONNX embeddings.
+# The bundled-embeddings default (#237) links ONNX Runtime via `ort`, whose
+# prebuilt binaries are glibc-only and don't work on Alpine/musl (and the
+# download path pulls in openssl-sys, absent here). A single static musl binary
+# is the right artifact for the Firecracker sandbox; FTS5 keyword recall works
+# out of the box, and dense/hybrid search can use an external embedder. For a
+# semantic-search-by-default image, use a glibc base (see issue/roadmap).
 FROM rust:1.96-alpine AS builder
 RUN apk add --no-cache musl-dev sqlite-dev
 WORKDIR /app
 COPY Cargo.toml Cargo.lock ./
 COPY src/ ./src/
-RUN cargo build --release && strip target/release/mimir
+COPY build.rs ./
+RUN cargo build --release --no-default-features && strip target/release/mimir
 
 FROM alpine:3.21
 RUN apk add --no-cache sqlite-libs