Dockerfileを書いて試してみる

実際のところ

https://hub.docker.com/_/alpine/:tilte=alpine on Docker hub
Docker公式が推してる軽量Linux*1、alpineのイメージを導入

$ docker pull alpine

次にDockerイメージをビルドするための作業環境を用意

$ mkdir dockerbuild
$ cd dockerbuild
$ touch Dockerfile

とりあえず何かやらせる

最初のDockerfileという事で、何か端末に表示するだけの簡単なのを。

Dockerfileの中身はこんな風になる。

FROM alpine:3.14
CMD printf "%10d" 1

作業環境にいる状態で、これをfoobarというタグ名でビルド

$ docker.exe build -t foobar .
[+] Building 1.6s (5/5) FINISHED
 => [internal] load build definition from Dockerfile                                                               0.0s
 => => transferring dockerfile: 75B                                                                                0.0s
 => [internal] load .dockerignore                                                                                  0.0s
 => => transferring context: 2B                                                                                    0.0s
 => [internal] load metadata for docker.io/library/alpine:3.14                                                     1.5s
 => CACHED [1/1] FROM docker.io/library/alpine:3.14@sha256:06b5d462c92fc39303e6363c65e074559f8d6b1363250027ed5053  0.0s
 => exporting to image                                                                                             0.0s
 => => exporting layers                                                                                            0.0s
 => => writing image sha256:ed515db3c32298c3908eb037fe034b8f80bbf3fba32665ec60f169ed561a2f91                       0.0s
 => => naming to docker.io/library/foobar                                                                          0.0s

Use 'docker scan' to run Snyk tests against images to find vulnerabilities and learn how to fix them

runでコンテナを作って実行。
"-it"はSTDINの有効化と、ttyの有効化。

$ docker run --name great_goat -it foobar
0000000001

対話型シェルを立ち上げる

先ほどの例だと実行するなり速攻でクローズするので、せめてシェルを立ち上げて何かできるようにする。

今度のDockerfileはENTRYPOINTというのを設定。

FROM alpine:3.14
ENTRYPOINT ["/bin/sh"]

$ docker.exe build -t fizzbuzz .
[+] Building 0.9s (5/5) FINISHED
 => [internal] load build definition from Dockerfile                                                               0.0s
 => => transferring dockerfile: 77B                                                                                0.0s
 => [internal] load .dockerignore                                                                                  0.0s
 => => transferring context: 2B                                                                                    0.0s
 => [internal] load metadata for docker.io/library/alpine:3.14                                                     0.8s
 => CACHED [1/1] FROM docker.io/library/alpine:3.14@sha256:06b5d462c92fc39303e6363c65e074559f8d6b1363250027ed5053  0.0s
 => exporting to image                                                                                             0.0s
 => => exporting layers                                                                                            0.0s
 => => writing image sha256:cb0566ca332cb4b321c6df0d459758c515736e5212d73971ee7a54b37fc632e0                       0.0s
 => => naming to docker.io/library/fizzbuzz                                                                        0.0s

Use 'docker scan' to run Snyk tests against images to find vulnerabilities and learn how to fix them

対話式でrunすると……ちゃんとshが実行される

$ docker run --name poor_pig -it fizzbuzz
/ #

さらに、イメージが実行状態であれば、別の端末で以下の様にやってやる事で新しいshを実行可能。

$docker exec -it poor_pig sh

参考もと

bash - Dockerfile CMD instruction will exit the container just after running it - Stack Overflow
How To Use docker exec to Run Commands in a Docker Container | DigitalOcean
How to open multiple terminals in docker? - Stack Overflow
Dockerfile reference | Docker Documentation
自分でイメージを構築 — Docker-docs-ja 1.13.RC ドキュメント

*1:なんと5Mbyte