Alpine Dockerfile Advantages of --no-cache Vs. rm /var/cache/apk/*

I think this is a design style. The essence of cache is to reuse, for example, multiple containers can mount the same cached file system without repeatedly downloading it from the network.

Can view the apline wiki: https://wiki.alpinelinux.org/wiki/Alpine_Linux_package_management#Local_Cache

The --no-cache option allows to not cache the index locally, which is useful for keeping containers small.

Literally it equals apk update in the beginning and rm -rf /var/cache/apk/* in the end.

Some example where we use --no-cache option:

$ 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 ..

Another example where we don't use --no-cache option:

$ 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'

As you can see both cases are valid. As for me, using --no-cache option is more elegant.