Enable Docker Remote API on Ubuntu

Client:
 Version:           18.09.0
 API version:       1.39
 Go version:        go1.10.4
 Git commit:        4d60db4
 Built:             Wed Nov  7 00:48:57 2018
 OS/Arch:           linux/amd64
 Experimental:      false

Server: Docker Engine - Community
 Engine:
  Version:          18.09.0
  API version:      1.39 (minimum version 1.12)
  Go version:       go1.10.4
  Git commit:       4d60db4
  Built:            Wed Nov  7 00:16:44 2018
  OS/Arch:          linux/amd64
  Experimental:     false
NAME="Ubuntu"
VERSION="16.04.5 LTS (Xenial Xerus)"
ID=ubuntu
ID_LIKE=debian
PRETTY_NAME="Ubuntu 16.04.5 LTS"
VERSION_ID="16.04"
HOME_URL="http://www.ubuntu.com/"
SUPPORT_URL="http://help.ubuntu.com/"
BUG_REPORT_URL="http://bugs.launchpad.net/ubuntu/"
VERSION_CODENAME=xenial
UBUNTU_CODENAME=xenial

参考

Docker – How do I enable the remote API for dockerd

Docker – Using systemd to control the Docker daemon

Dockerデーモンソケットを保護します。 Dockerのドキュメント

結論

# sytemctl edit docker 

[Service]
ExecStart=
ExecStart=/usr/bin/dockerd -H tcp://{{ 0.0.0.0 or RESTRICT_IP }}:2375 -H unix:///var/run/docker.sock
$ docker version 
Client: Docker Engine - Community
 Version:           18.09.0-ce-beta1
 API version:       1.39
 Go version:        go1.10.4
 Git commit:        78a6bdb
 Built:             Thu Sep  6 22:41:53 2018
 OS/Arch:           darwin/amd64
 Experimental:      false

Server: Docker Engine - Community
 Engine:
  Version:          18.09.0
  API version:      1.39 (minimum version 1.12)
  Go version:       go1.10.4
  Git commit:       4d60db4
  Built:            Wed Nov  7 00:16:44 2018
  OS/Arch:          linux/amd64
  Experimental:     false

補足

port 2375 はipv4
port 2376 はtls/ssl ポート
Dockerデーモンソケットを保護します。 Dockerのドキュメント