Add openssh package

* `acme.sh`'s `ssh.sh` is probably one of the hooks that's most versatile
* By default, it's not installed on `alpine` docker images and therefore is lacking in the `acme.sh` docker image
* This change adds the `openssh` package and therefore the `ssh` and associated commands

Dockerfile

@@ -3,6 +3,7 @@ FROM alpine:3.10
 RUN apk update -f \
   && apk --no-cache add -f \
   openssl \
+  openssh \
   coreutils \
   bind-tools \
   curl \