fix(docker): copy synocli source into build context

[matonb]

Summary

docker build currently fails on an unmodified checkout of main. The
build stage's make target compiles both bin/synology-csi-driver and
bin/synocli (since build: bin/synology-csi-driver bin/synocli was
introduced in the CI-workflow commit), but the Dockerfile only ever
COPYs main.go and pkg/ into the builder image - synocli/ was
never added to the COPY list, so the in-container make fails:

Compiling bin/synocli…
stat /go/src/synok8scsiplugin/synocli: directory not found
make: *** [Makefile:42: bin/synocli] Error 1

This is why it hasn't been caught by CI: .github/workflows/ci.yaml
only runs make build directly on the runner (where synocli/ is
already present in the full checkout) and never invokes docker build,
so the two build paths diverged silently. Reproducible on a clean
checkout of main with just docker build -t test ..

Fix

Add the missing COPY synocli ./synocli alongside the existing
COPY main.go . / COPY pkg ./pkg lines.

Test plan

• docker build -t synology-csi:test . fails on unmodified main with the error above
• Same build succeeds with this one-line change, producing a working image (arm64), which I've since run in a live cluster

[matonb]

Hi — noticed daae1c1 on main lands this exact same one-line fix (COPY synocli ./synocli) that this PR already proposed on 2026-07-20. Happy to close this PR in favor of that commit — just flagging so it doesn't sit open unnecessarily. Also have #146 and #147 open if anyone gets a chance to look.