update Dockerfile for opam 2.2 and ocaml 5.3

In particular io-page-unix needs recent opam

Signed-off-by: David Scott <[email protected]>

Author: djs55

Dockerfile

@@ -1,11 +1,10 @@
-FROM ocaml/opam:alpine-3.15-ocaml-4.14 as build
-RUN opam update
+FROM ocaml/opam:alpine-ocaml-5.3 AS build
+RUN opam-2.2 update
 
 ADD . /home/opam/vpnkit
-RUN opam pin add vpnkit /home/opam/vpnkit -n
-RUN opam depext vpnkit -y
+RUN opam-2.2 pin add vpnkit file:///home/opam/vpnkit -n
 
-RUN opam install vpnkit -y
+RUN opam-2.2 install vpnkit -y
 
 FROM alpine:latest
-COPY --from=build /home/opam/.opam/4.14/bin/vpnkit /vpnkit
+COPY --from=build /home/opam/.opam/5.3/bin/vpnkit /vpnkit