Add reproducible unprivileged Linux build and mount checks

scripts/check-linux-container.sh

@@ -0,0 +1,26 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+test "$(uname -s)" = Linux
+test "$(id -u)" != 0
+printf 'Linux checks: uid=%s architecture=%s\n' "$(id -u)" "$(uname -m)"
+rustc --version
+cargo --version
+
+# Only source inputs are copied. Cargo output and all fixtures disappear with
+# the container; the host checkout is read-only and no personal tree is scanned.
+check_root=$(mktemp -d /tmp/sized-check.XXXXXX)
+cp /source/Cargo.toml /source/Cargo.lock "$check_root/"
+cp -R /source/src /source/tests /source/benches "$check_root/"
+cd "$check_root"
+cargo fmt --all -- --check
+cargo test --locked
+cargo test --locked --test linux_mount -- --ignored
+cargo clippy --locked --all-targets -- -D warnings
+cargo build --locked --release
+
+fixture_root=$(mktemp -d /tmp/sized-release.XXXXXX)
+printf 'Linux release smoke test\n' > "$fixture_root/payload"
+./target/release/sized --version
+./target/release/sized "$fixture_root" --threads 2 --one-file-system --apparent --format json
+printf 'Linux checks passed.\n'