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

@@ -31,7 +31,7 @@ RUN opam install . --destdir /home/opam/install-prefix --locked
 FROM alpine:3.13 as client
 
 RUN apk update \
-  && apk add ncurses-libs libev dumb-init openssl \
+  && apk add ncurses-libs libev dumb-init openssl lsof \
   && addgroup learn-ocaml \
   && adduser learn-ocaml -DG learn-ocaml
 
@@ -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-client

@@ -51,7 +51,7 @@ 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 openssl \
+  && apk add ncurses-libs libev dumb-init 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