Alpine Dockerfile的优点--no-cache Vs. rm / var / cache / apk / *

Question

当使用Alpine图像创建Dockerfiles时，我经常看到使用apk --no-cache，有时它会被提交，而我会看到rm /var/cache/apk/*。

我很想知道利用--no-cache消除了以后做rm /var/cache/apk/*的需要。我还想知道一种风格是否比另一种更受青睐。

Answer 1

--no-cache选项允许不在本地缓存索引，这对于保持容器较小很有用。

从字面上看，它最初等于apk update，最后等于rm -rf /var/cache/apk/*。

我们使用--no-cache选项的一些示例：

$ docker run -ti alpine:3.7
/ # apk add nginx
WARNING: Ignoring APKINDEX.70c88391.tar.gz: No such file or directory
WARNING: Ignoring APKINDEX.5022a8a2.tar.gz: No such file or directory
ERROR: unsatisfiable constraints:
  nginx (missing):
    required by: world[nginx]
/ # 
/ # apk add --no-cache nginx
fetch http://dl-cdn.alpinelinux.org/alpine/v3.7/main/x86_64/APKINDEX.tar.gz
fetch http://dl-cdn.alpinelinux.org/alpine/v3.7/community/x86_64/APKINDEX.tar.gz
(1/2) Installing pcre (8.41-r1)
(2/2) Installing nginx (1.12.2-r3)
Executing nginx-1.12.2-r3.pre-install
Executing busybox-1.27.2-r7.trigger
OK: 6 MiB in 13 packages
/ # 
/ # ls -la /var/cache/apk/
total 8
drwxr-xr-x    2 root     root          4096 Jan  9 19:37 .
drwxr-xr-x    5 root     root          4096 Mar  5 20:29 ..

我们不使用--no-cache选项的另一个例子：

$ docker run -ti alpine:3.7
/ # apk add nginx
WARNING: Ignoring APKINDEX.70c88391.tar.gz: No such file or directory
WARNING: Ignoring APKINDEX.5022a8a2.tar.gz: No such file or directory
ERROR: unsatisfiable constraints:
  nginx (missing):
    required by: world[nginx]
/ # 
/ # apk update
fetch http://dl-cdn.alpinelinux.org/alpine/v3.7/main/x86_64/APKINDEX.tar.gz
fetch http://dl-cdn.alpinelinux.org/alpine/v3.7/community/x86_64/APKINDEX.tar.gz
v3.7.0-107-g15dd6b8ab3 [http://dl-cdn.alpinelinux.org/alpine/v3.7/main]
v3.7.0-105-g4b8b158c40 [http://dl-cdn.alpinelinux.org/alpine/v3.7/community]
OK: 9048 distinct packages available
/ # 
/ # apk add nginx
(1/2) Installing pcre (8.41-r1)
(2/2) Installing nginx (1.12.2-r3)
Executing nginx-1.12.2-r3.pre-install
Executing busybox-1.27.2-r7.trigger
OK: 6 MiB in 13 packages
/ # 
/ # ls -la /var/cache/apk/
total 1204
drwxr-xr-x    2 root     root          4096 Mar  5 20:31 .
drwxr-xr-x    6 root     root          4096 Mar  5 20:31 ..
-rw-r--r--    1 root     root        451508 Mar  3 00:30 APKINDEX.5022a8a2.tar.gz
-rw-r--r--    1 root     root        768680 Mar  5 09:39 APKINDEX.70c88391.tar.gz
/ # 
/ # rm -vrf /var/cache/apk/*
removed '/var/cache/apk/APKINDEX.5022a8a2.tar.gz'
removed '/var/cache/apk/APKINDEX.70c88391.tar.gz'

如您所见，两种情况都有效。至于我，使用--no-cache选项更优雅。