docker: install static zstd for OCaml 5.x static linking

OCaml 5.x compiler-libs link against zstd; combined with the static link
(-ccopt -static) the Alpine build needs libzstd.a, which isn't present by
default. Add zstd-static to both Dockerfiles. Fixes 'ld: cannot find -lzstd'
in the bin-image and Dockerfile.test builds.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: David Scott <dave@recoil.org>

Author: djs55

Dockerfile

@@ -1,5 +1,8 @@
 FROM ocaml/opam:alpine-ocaml-5.3 AS build
 RUN opam update
+# Static zstd is needed because OCaml 5.x compiler-libs link against zstd and
+# the binary is statically linked (see src/bin/config/discover.ml).
+RUN sudo apk add --no-cache zstd-static
 
 ADD . /home/opam/vpnkit
 RUN opam pin add vpnkit /home/opam/vpnkit --kind=path -n

Dockerfile.test

@@ -1,5 +1,8 @@
 FROM ocaml/opam:alpine-ocaml-5.3 AS build
 RUN opam update
+# Static zstd is needed because OCaml 5.x compiler-libs link against zstd and
+# the binary is statically linked (see src/bin/config/discover.ml).
+RUN sudo apk add --no-cache zstd-static
 
 ADD . /home/opam/vpnkit
 RUN sudo chown opam:opam -R vpnkit