# ssh -i id_rsa -MS ./ctrl -fTN -p22 -l root example.com
# ssh -S ./ctrl -O check example.com
Master running (pid=23384)

# ssh -i id_rsa -MS ./ctrl -fTN -p22 -l root example.com
# ssh -S ./ctrl -O check example.com
Control socket connect(./ctrl): Connection refused

# apk list | grep ssh
WARNING: Ignoring APKINDEX.b89edf6e.tar.gz: No such file or directory
WARNING: Ignoring APKINDEX.737f7e01.tar.gz: No such file or directory
openssh-client-7.9_p1-r5 x86_64 {openssh} (BSD) [installed]
openssh-keygen-7.9_p1-r5 x86_64 {openssh} (BSD) [installed]
autossh-1.4f-r0 x86_64 {autossh} (BSD) [installed]

$ docker --version
Docker version 18.09.2, build 6247962

docker --version
Docker version 18.09.3, build 774a1f4