Add a module-scoped `shared_container` fixture to tests/docker/conftest.py
that boots one `sleep infinity` container per test module and tears it
down at module exit. Convert read-only tests that previously used
`docker run --rm --entrypoint sh/cat/test/su` (bypassing s6 to check
static image properties) or `docker run -d` + `docker exec` (starting
identical containers per test) to use `docker exec` on the shared
container instead.
Converted files:
test_immutable_install_permissions.py — 2 throwaway runs → 2 execs
test_license_file_present.py — 1 throwaway run → 1 exec
test_tini_compat_shim.py — 1 throwaway run → 1 exec
test_tui_prebuilt_bundle.py — 2 throwaway runs → 2 execs
test_dump_build_sha.py — 2 throwaway runs → 2 execs
test_immutable_install.py — 3 detached runs → 1 shared + 1 isolated
test_dashboard.py — 2 detached runs → 0 (use shared)
Local profiling shows docker run calls in these 7 files dropped from
~25 to 7 (the 7 are shared_container boots per module + the one test
that needs a restart). Each eliminated `docker run` was paying 1-9s
of s6 cont-init startup; the replacement `docker exec` calls average
0.10s — an ~50x speedup per operation.
Tests that mutate state (restarts, config changes, gateway starts)
still use their own containers via `container_name` + `start_container`.
