add SSH client to Docker image

Author: joonas-fi

Dockerfile

@@ -1,5 +1,8 @@
 FROM alpine:3.11.3
 
+# add SSH client
+RUN apk add --update openssh
+
 CMD ["holepunch-server", "server", "--http-reverse-proxy", "--sshd-websocket"]
 
 ADD rel/holepunch-server_linux-amd64 /usr/local/bin/holepunch-server