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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.
&& apk add bash tells Docker to install bash into the image. apk stands for Alpine Linux package manager. If you're using a Linux base image in a flavor other than Alpine, then you'd install packages with RUN apt-get instead of apk.
What Is an APK File? A file with the APK file extension is an Android Package file that's used to distribute apps on Google's Android operating system. APK files are saved in the ZIP format and are typically downloaded directly to Android devices, usually via Google Play, but can also be found on other websites.
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.
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
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