added link to list of potential alternatives for socat

Dockerfile

@@ -12,6 +12,7 @@ FROM alpine:3.11 as service
 ARG CPANM_PACKAGE_VERSION=1.7044-r1
 ARG GNUPG_PACKAGE_VERSION=2.2.19-r0
 ARG PERL_MIME_TOOLS_PACKAGE_VERSION=5.509-r1
+# potential alternatives: https://unix.stackexchange.com/a/315470/155174
 ARG SOCAT_PACKAGE_VERSION=1.7.3.3-r1
 ARG TINI_PACKAGE_VERSION=0.18.0-r0
 ENV GNUPGHOME=/gnupg_home