fix: runtime error in Docker packaging

* Add missing Alpine package
  href: https://pkgs.alpinelinux.org/package/v3.7/main/x86_64/openssl

* Avoid runtime errors:
  Error loading shared library libssl.so.1.0.0: No such file or directory (needed by /usr/bin/learn-ocaml)
  Error loading shared library libcrypto.so.1.0.0: No such file or directory (needed by /usr/bin/learn-ocaml)
  […]

* Ensure this patch is future-proof (instead of adding "libssl1.0"), see:
  eclipse/mosquitto#1711 (comment)

Dockerfile

@@ -31,7 +31,7 @@ RUN opam install . --destdir /home/opam/install-prefix --locked
 FROM alpine:3.7 as client
 
 RUN apk update \
-  && apk add ncurses-libs libev dumb-init \
+  && apk add ncurses-libs libev dumb-init openssl \
   && addgroup learn-ocaml \
   && adduser learn-ocaml -DG learn-ocaml
 
@@ -53,7 +53,7 @@ LABEL org.opencontainers.image.vendor="The OCaml Software Foundation"
 FROM alpine:3.7 as program
 
 RUN apk update \
-  && apk add ncurses-libs libev dumb-init git \
+  && apk add ncurses-libs libev dumb-init git openssl \
   && 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 \
+  && apk add ncurses-libs libev dumb-init openssl \
   && addgroup learn-ocaml \
   && adduser learn-ocaml -DG learn-ocaml