tl;dr

Host の Docker を使って Docker Container 内で Docker を使う方法
Docker in Docker とは少し違う
- この場合、Docker Image に Docker を再度 Install する
Docker Sibling っていうらしい
Ubuntu だと楽だったが Alpine だと少し詰まった

Environment

Ubuntu: 16.04
Docker: 18.09.1
docker-compose: 1.23.1

$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 16.04.5 LTS
Release:    16.04
Codename:   xenial
$ docker --version
Docker version 18.09.1, build 4c52b90
$ which docker
/usr/bin/docker
$ docker-compose -v
docker-compose version 1.23.1, build b02f1306

Ubuntu Image

簡単な方の例として Ubuntu Image を使った Docker Sibling を先にやってみる
Host の下記のものを mount しなければならない
- /var/run/docker.sock
- /usr/bin/docker
- /var/lib/docker
apt で以下の library を Install する
- libltdl7

docker-compose を使う場合は下記の docker-compose.yml を使う

FROM ubuntu:xenial

RUN apt update && \
    apt install -y \
    libltdl7 && \
    apt clean && \
    rm -rf /var/lib/apt/lists/*

version: "3"
services:
  sibling:
    build:
      context: .
      dockerfile: Dockerfile
    volumes:
      - /var/run/docker.sock:/var/run/docker.sock
      - /usr/bin/docker:/usr/bin/docker
      - /var/lib/docker:/var/lib/docker
    command: ["tail", "-f", "/dev/null"]

上のファイルが置いてある dir で

$ docker-compose up -d --build
Creating network "ubuntu_default" with the default driver
Building sibling
Step 1/2 : FROM ubuntu:xenial
 ---> 7e87e2b3bf7a
Step 2/2 : RUN apt update &&     apt install -y     libltdl7 &&     apt clean &&     rm -rf /var/lib/apt/lists/*
 ---> Running in ba4a3656aa54

...

Removing intermediate container ba4a3656aa54
 ---> 1d55ed6f9167
Successfully built 1d55ed6f9167
Successfully tagged ubuntu_sibling:latest
Creating ubuntu_sibling_1_6d8a96c7808b ... done

$ docker-compose exec sibling bash
root@1c72523ac4ca:/#

Docker container 内で docker command を叩いてみる

root@1c72523ac4ca:/# docker --version
Docker version 18.09.1, build 4c52b90
root@1c72523ac4ca:/# docker ps
CONTAINER ID        IMAGE                 COMMAND               CREATED              STATUS              PORTS                  NAMES
1c72523ac4ca        ubuntu:xenial         "tail -f /dev/null"   About a minute ago   Up About a minute                          ubuntu_sibling_1_c6a532a1bf85
root@1c72523ac4ca:/# docker run hello-world
Unable to find image 'hello-world:latest' locally
latest: Pulling from library/hello-world
1b930d010525: Pull complete
Digest: sha256:2557e3c07ed1e38f26e389462d03ed943586f744621577a99efb77324b0fe535
Status: Downloaded newer image for hello-world:latest

Hello from Docker!
This message shows that your installation appears to be working correctly.

To generate this message, Docker took the following steps:
 1. The Docker client contacted the Docker daemon.
 2. The Docker daemon pulled the "hello-world" image from the Docker Hub.
    (amd64)
 3. The Docker daemon created a new container from that image which runs the
    executable that produces the output you are currently reading.
 4. The Docker daemon streamed that output to the Docker client, which sent it
    to your terminal.

To try something more ambitious, you can run an Ubuntu container with:
 $ docker run -it ubuntu bash

Share images, automate workflows, and more with a free Docker ID:
 https://hub.docker.com/

For more examples and ideas, visit:
 https://docs.docker.com/get-started/

動いた。

Alpine Image

Mount するものは同じ
- /var/run/docker.sock
- /usr/bin/docker
- /var/lib/docker
docker-compose.yml も同じ
apk で以下の library を Install する
- libltdl
- libc6-compat

libc6-compat を install しないで container を起動した場合、以下の症状になる

/ # docker --version
sh: docker: not found
/ # which docker
/usr/bin/docker
/ # echo $PATH
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
/ # /usr/bin/docker
sh: /usr/bin/docker: not found

libc6-compat を install した場合は、

FROM alpine

RUN apk --update add --no-cache \
    libltdl \
    libc6-compat

ubuntu@dh236:~/docker/ubuntu$ docker-compose up -d --build
Creating network "ubuntu_default" with the default driver
Building sibling
Step 1/2 : FROM alpine
 ---> 3f53bb00af94
Step 2/2 : RUN apk --update add --no-cache     libc6-compat     libltdl
 ---> Running in bdadee15e55a

...

Removing intermediate container bdadee15e55a
 ---> 74a3b17a7bfa
Successfully built 74a3b17a7bfa
Successfully tagged ubuntu_sibling:latest
Creating ubuntu_sibling_1_5c2a7d4c159a ... done
ubuntu@dh236:~/docker/ubuntu$ docker-compose exec sibling sh
/ #

/ # docker --version
Docker version 18.09.1, build 4c52b90
/ # docker ps
CONTAINER ID        IMAGE                 COMMAND               CREATED             STATUS              PORTS                  NAMES
a43a0d067029        ubuntu_sibling        "tail -f /dev/null"   45 seconds ago      Up 44 seconds                              ubuntu_sibling_1_fa2513c9a2e7
/ # docker run hello-world

Hello from Docker!
This message shows that your installation appears to be working correctly.

To generate this message, Docker took the following steps:
 1. The Docker client contacted the Docker daemon.
 2. The Docker daemon pulled the "hello-world" image from the Docker Hub.
    (amd64)
 3. The Docker daemon created a new container from that image which runs the
    executable that produces the output you are currently reading.
 4. The Docker daemon streamed that output to the Docker client, which sent it
    to your terminal.

To try something more ambitious, you can run an Ubuntu container with:
 $ docker run -it ubuntu bash

Share images, automate workflows, and more with a free Docker ID:
 https://hub.docker.com/

For more examples and ideas, visit:
 https://docs.docker.com/get-started/

動いた。ちなみに alpine base の image のサイズは、

$ docker images
REPOSITORY                          TAG                  IMAGE ID            CREATED             SIZE
alpine_sibling                      latest               74a3b17a7bfa        2 minutes ago       4.47MB

まとめ

libltdl だけだと変な挙動になった
- libc6-compat が必要だった

虚無ありき

うるせーーーしらねーーー

Alpine Image を用いた Docker 横付け(Sibling)

tl;dr

Environment

Ubuntu Image

Alpine Image

まとめ