【エラー】 Docker build時のエラー(最新版のimageをpullして解決)

# 概要
* Dockerの勉強中にbuidでこけたのでその対処をしたので、その備忘。
* 解決方法としては単にベースのイメージが古かったので新しくpullしてbuildして終了。

# 状況と対処
## buildしたDockerfile
Dockerfileは非常にシンプルで、ubuntu:latestをベースにパッケージをインストールしている。

```docker:Dockerfile
FROM ubuntu:latest
RUN apt-get update && apt-get install -y \ 
	curl \
	nginx
```

## エラー
セキュリティーが怪しそうとのことなのでおそらくdocker imageが古い?

```
$ docker build .
Sending build context to Docker daemon  6.656kB
Step 1/2 : FROM ubuntu:latest
 ---> 9140108b62dc
Step 2/2 : RUN apt-get update && apt-get install -y 	curl 	nginx
 ---> Running in b37d029342e6
Get:1 http://security.ubuntu.com/ubuntu focal-security InRelease [109 kB]
Get:2 http://archive.ubuntu.com/ubuntu focal InRelease [265 kB]
Get:3 http://security.ubuntu.com/ubuntu focal-security/restricted amd64 Packages [109 kB]
Ign:4 http://security.ubuntu.com/ubuntu focal-security/main amd64 Packages
Ign:5 http://security.ubuntu.com/ubuntu focal-security/universe amd64 Packages
Get:6 http://security.ubuntu.com/ubuntu focal-security/multiverse amd64 Packages [741 B]
Get:4 http://security.ubuntu.com/ubuntu focal-security/main amd64 Packages [516 kB]
Err:4 http://security.ubuntu.com/ubuntu focal-security/main amd64 Packages
  File has unexpected size (515215 != 515734). Mirror sync in progress? [IP: 91.189.91.39 80]
  Hashes of expected file:
   - Filesize:515734 [weak]
   - SHA256:6edc1a6f67f0e67da44c8aa3455144dba75d1f11ecd18bb9881634574c4a5b10
   - SHA1:8639b548198806c25d72f9dd9e9a9d6f86dcb895 [weak]
   - MD5Sum:b29deacaed369971375c55ea72765d06 [weak]
  Release file created at: Wed, 06 Jan 2021 13:47:03 +0000
Get:7 http://archive.ubuntu.com/ubuntu focal-updates InRelease [114 kB]
Get:5 http://security.ubuntu.com/ubuntu focal-security/universe amd64 Packages [646 kB]
Err:5 http://security.ubuntu.com/ubuntu focal-security/universe amd64 Packages

Get:8 http://archive.ubuntu.com/ubuntu focal-backports InRelease [101 kB]
Get:9 http://archive.ubuntu.com/ubuntu focal/restricted amd64 Packages [33.4 kB]
Get:10 http://archive.ubuntu.com/ubuntu focal/universe amd64 Packages [11.3 MB]
Get:11 http://archive.ubuntu.com/ubuntu focal/multiverse amd64 Packages [177 kB]
Get:12 http://archive.ubuntu.com/ubuntu focal/main amd64 Packages [1275 kB]
Get:13 http://archive.ubuntu.com/ubuntu focal-updates/multiverse amd64 Packages [30.0 kB]
Get:14 http://archive.ubuntu.com/ubuntu focal-updates/universe amd64 Packages [897 kB]
Get:15 http://archive.ubuntu.com/ubuntu focal-updates/restricted amd64 Packages [145 kB]
Get:16 http://archive.ubuntu.com/ubuntu focal-updates/main amd64 Packages [916 kB]
Get:17 http://archive.ubuntu.com/ubuntu focal-backports/universe amd64 Packages [4250 B]
Fetched 15.5 MB in 12min 47s (20.2 kB/s)
Reading package lists...
E: Failed to fetch http://security.ubuntu.com/ubuntu/dists/focal-security/main/binary-amd64/Packages.gz  File has unexpected size (515215 != 515734). Mirror sync in progress? [IP: 91.189.91.39 80]
   Hashes of expected file:
    - Filesize:515734 [weak]
    - SHA256:6edc1a6f67f0e67da44c8aa3455144dba75d1f11ecd18bb9881634574c4a5b10
    - SHA1:8639b548198806c25d72f9dd9e9a9d6f86dcb895 [weak]
    - MD5Sum:b29deacaed369971375c55ea72765d06 [weak]
   Release file created at: Wed, 06 Jan 2021 13:47:03 +0000
E: Failed to fetch http://security.ubuntu.com/ubuntu/dists/focal-security/universe/binary-amd64/Packages.gz
E: Some index files failed to download. They have been ignored, or old ones used instead.
The command '/bin/sh -c apt-get update && apt-get install -y 	curl 	nginx' returned a non-zero code: 100
```

## 解決

docker imageを確認すると3か月前だったのでとりあえず最新版のimageをpullしてそれを元にbuildしたらいけた。

```
$ docker images
REPOSITORY                     TAG       IMAGE ID       CREATED         SIZE
ubuntu                         latest    9140108b62dc   3 months ago    72.9MB

$ docker pull ubuntu:latest
latest: Pulling from library/ubuntu
da7391352a9b: Pull complete
14428a6d4bcd: Pull complete
2c2d948710f2: Pull complete
Digest: sha256:c95a8e48bf88e9849f3e0f723d9f49fa12c5a00cfc6e60d2bc99d87555295e4c
Status: Downloaded newer image for ubuntu:latest
docker.io/library/ubuntu:latest
```

もう一度docker buildしてみたら成功した。

```
$ docker build .
Sending build context to Docker daemon  6.656kB
Step 1/3 : FROM ubuntu:latest
 ---> f643c72bc252
Step 2/3 : RUN apt-get update
 ---> Running in 526d97fff024
Get:1 http://archive.ubuntu.c

略

Successfully built a4360c2d987b
```