fix(docker): repair the Rust pin and build a real musl binary

.github/workflows/ci.yml

@@ -9,6 +9,7 @@ on:
       - 'Cargo.toml'
       - 'Cargo.lock'
       - 'deny.toml'
+      - 'docker/**'
       - '.github/workflows/ci.yml'
   pull_request:
     branches: [ main ]
@@ -18,6 +19,7 @@ on:
       - 'Cargo.toml'
       - 'Cargo.lock'
       - 'deny.toml'
+      - 'docker/**'
       - '.github/workflows/ci.yml'
 
 # Cancel outdated workflow runs
@@ -301,6 +303,67 @@ jobs:
         working-directory: fuzz
         run: cargo audit
 
+  # Job 5b: Container images must build, and the binary inside must run.
+  #
+  # Nothing validated docker/Dockerfile before: packages.yml only fires on a
+  # published release or a manual dispatch. Two defects accumulated unnoticed --
+  # the builder pinned a Rust below the workspace MSRV, and the alpine stage
+  # copied a Debian glibc binary into a musl image, producing a container in
+  # which prtip could not exec at all.
+  #
+  # `--version` is the check that matters. A glibc/musl mismatch builds a
+  # perfectly good-looking image and only fails when something tries to run it.
+  docker:
+    name: Docker Images
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Build Debian runtime image
+        uses: docker/build-push-action@v6
+        with:
+          context: .
+          file: docker/Dockerfile
+          target: runtime
+          tags: prtip:ci-runtime
+          load: true
+          cache-from: type=gha
+          cache-to: type=gha,mode=max
+
+      - name: Build Alpine image
+        uses: docker/build-push-action@v6
+        with:
+          context: .
+          file: docker/Dockerfile
+          target: alpine
+          tags: prtip:ci-alpine
+          load: true
+          cache-from: type=gha
+          cache-to: type=gha,mode=max
+
+      - name: Binary runs in both images
+        run: |
+          for image in prtip:ci-runtime prtip:ci-alpine; do
+            echo "::group::$image"
+            docker run --rm "$image" --version
+            echo "::endgroup::"
+          done
+
+      - name: Alpine binary is musl-linked, not glibc
+        run: |
+          # Guards against a regression to copying the Debian binary: that
+          # produced an image which built fine and failed on first exec.
+          interp=$(docker run --rm --entrypoint sh prtip:ci-alpine \
+                     -c 'ldd /usr/local/bin/prtip 2>&1' | head -1)
+          echo "interpreter: $interp"
+          case "$interp" in
+            *ld-musl*) echo "OK: musl-linked" ;;
+            *) echo "FAIL: expected a musl interpreter, got: $interp"; exit 1 ;;
+          esac
+
   # Job 6: MSRV (Minimum Supported Rust Version) check
   msrv:
     name: MSRV Check (1.88)

CHANGELOG.md

@@ -261,6 +261,28 @@ along with every unmaintained/unsound warning. `cargo audit` and
   third-party-material statement moved there, because `LICENSE` has to be the
   licence text and nothing else for both of the reasons above. Nothing was
   dropped in the move.
+- **The Docker build was broken by the MSRV raise.** `docker/Dockerfile` pinned
+  `ARG RUST_VERSION=1.85`, below the workspace's new 1.88 floor, so the builder
+  stage could no longer compile the project at all. Nothing caught it because
+  `packages.yml` — the only workflow that touches the Dockerfile — runs solely on
+  a published release or a manual dispatch.
+- **The Alpine image never worked.** Its stage copied the Debian-built **glibc**
+  binary into a **musl** image. Alpine ships no `/lib64/ld-linux-x86-64.so.2`, so
+  the kernel failed the exec with a bare "no such file or directory" — the image
+  built cleanly and `prtip` could not run in it, which the stage's own
+  `HEALTHCHECK` would have failed on every start. The comment in place
+  ("For now, use the glibc binary (requires compatibility layer or rebuild)")
+  acknowledged it. There is now a real `musl-builder` stage building natively on
+  `rust:1.88-alpine` with `libpcap-dev`, `openssl-dev` and a C toolchain from
+  apk, which is far simpler than assembling a musl sysroot on Debian. Verified:
+  the binary reports `prtip 1.0.0` inside the image and links
+  `/lib/ld-musl-x86_64.so.1`; the image is 26.9 MB against the Debian variant's
+  97 MB.
+- **CI now builds both container images and runs the binary in each.** A
+  glibc/musl mismatch produces a perfectly good-looking image that only fails
+  when something tries to execute it, so the gate asserts `--version` succeeds
+  and that the Alpine binary's interpreter is musl. `ci.yml` path filters now
+  include `docker/**`.
 - **Seven broken internal documentation links** in
   `to-dos/PHASE-5/SPRINT-5.5.5-TODO.md` and `SPRINT-5.5.6-TODO.md`. They pointed
   at `../docs/...` and `../benchmarks/...` from `to-dos/PHASE-5/`, which resolves

docker/Dockerfile

@@ -9,7 +9,10 @@
 #   docker run --rm --net=host --cap-add=NET_RAW prtip -sS -p 80,443 192.168.1.1
 #   docker run -it --rm --net=host --cap-add=NET_RAW prtip --tui
 
-ARG RUST_VERSION=1.85
+# Must be >= the workspace `rust-version` in Cargo.toml (1.88). A pin below
+# the MSRV fails the build with "requires rustc 1.88 or newer"; nothing in CI
+# caught the drift because packages.yml only runs on releases.
+ARG RUST_VERSION=1.88
 ARG VERSION=1.0.0
 
 # ==============================================================================
@@ -104,6 +107,34 @@ HEALTHCHECK --interval=30s --timeout=5s --start-period=5s --retries=3 \
 ENTRYPOINT ["prtip"]
 CMD ["--help"]
 
+# ==============================================================================
+# Stage 2b: musl builder (for the Alpine variant)
+# ==============================================================================
+# Built inside Alpine rather than cross-compiled from Debian. `pcap` links
+# against libpcap, and `mlua` builds a vendored Lua, so both need a C toolchain
+# and headers that match the target libc. Building natively on Alpine gets musl
+# versions of both from apk, which is far simpler than assembling a musl
+# sysroot on Debian.
+FROM rust:${RUST_VERSION}-alpine AS musl-builder
+
+WORKDIR /build
+
+RUN apk add --no-cache \
+    libpcap-dev \
+    musl-dev \
+    pkgconfig \
+    build-base \
+    linux-headers \
+    openssl-dev \
+    openssl-libs-static
+
+COPY Cargo.toml Cargo.lock ./
+COPY crates/ crates/
+
+RUN cargo build --release --locked
+
+RUN strip /build/target/release/prtip
+
 # ==============================================================================
 # Stage 3: Alpine variant (smaller image, musl libc)
 # ==============================================================================
@@ -123,12 +154,13 @@ RUN apk add --no-cache \
 # Create non-root user
 RUN adduser -D -s /bin/sh prtip
 
-# Note: For Alpine, you need to build with musl target
-# This stage expects a pre-built musl binary
-# COPY --from=builder /build/target/x86_64-unknown-linux-musl/release/prtip /usr/local/bin/prtip
-
-# For now, use the glibc binary (requires compatibility layer or rebuild)
-COPY --from=builder /build/target/release/prtip /usr/local/bin/prtip
+# Binary comes from the musl builder stage above, not from `builder`.
+# Copying the Debian-built glibc binary here produced an image in which
+# `prtip` could not execute at all -- musl provides no
+# /lib64/ld-linux-x86-64.so.2, so the kernel fails the exec with a bare
+# "no such file or directory". The HEALTHCHECK below would have caught it
+# on every run of this image.
+COPY --from=musl-builder /build/target/release/prtip /usr/local/bin/prtip
 
 COPY README.md /usr/share/doc/prtip/
 COPY LICENSE /usr/share/doc/prtip/