Explanation of the “–update add” command for Alpine Linux

See https://github.com/gliderlabs/docker-alpine/pull/503

apk –update flag is really –update-cache.

Apk uses getopt_long (3),
https://github.com/alpinelinux/apk-tools/blob/v2.10.3/src/apk.c#L574

So, –update flag is only abbreviated from –update-cache by getopt_long.

Long option names may be abbreviated if the abbreviation is unique or
is an exact match for some defined option.