fix(docker): Add missing alpine package (lsof)

This makes it possible to fix the error:
```
Waiting for process 9 to terminate...  0 Error: process didn't terminate in time
```
triggered by either `--serve-during-build` or `--replace` CLI options.

Dockerfile

@@ -48,7 +48,7 @@ ENTRYPOINT ["dumb-init","/usr/bin/learn-ocaml-client"]
 FROM alpine:3.13 as program
 
 RUN apk update \
-  && apk add ncurses-libs libev dumb-init git openssl \
+  && apk add ncurses-libs libev dumb-init git openssl lsof \
   && addgroup learn-ocaml \
   && adduser learn-ocaml -DG learn-ocaml
 

Dockerfile.test-server

@@ -54,7 +54,7 @@ LABEL org.label-schema.build-date="${BUILD_DATE}" \
   org.label-schema.schema-version="1.0"
 
 RUN apk update \
-  && apk add ncurses-libs libev dumb-init git openssl \
+  && apk add ncurses-libs libev dumb-init git openssl lsof \
   && addgroup learn-ocaml \
   && adduser learn-ocaml -DG learn-ocaml
 

learn-ocaml.opam

@@ -17,6 +17,9 @@ license: "MIT"
 homepage: "https://github.com/ocaml-sf/learn-ocaml"
 bug-reports: "https://github.com/ocaml-sf/learn-ocaml/issues"
 dev-repo: "git+https://github.com/ocaml-sf/learn-ocaml"
+depexts: [
+  ["lsof"] {os-distribution = "alpine"}
+]
 depends: [
   "asak" { >= "0.4"}
   "base64"