Feature: install coreutils

[jimvandervoort]

Hello,

First, thank you @nicolaka for creating and maintaining this repo. It's been a life safer countless times :)

Recently I noticed coreutils is missing, I tried to run:

tshark \
  -l \
  -i eth0 \
  -f "tcp port 9000 or tcp port 9443 or tcp port 9030" \
  -o tls.keylog_file:/sslkeys/client-keys.log \
  -Y "http.request or http.response" \
  -T fields \
  -e tcp.payload \
  | stdbuf -o0 xxd -r -p

And noticed stdbuf is not installed.

Since this is image is meant for debugging I think it's nice to have the full coreutils installed instead of just the very basic ones that come with Alpine.

Note this is a breaking change since it replaces some tools like tr. The busybox tr has fewer features than the tr from coreutils. Other tools might also behave differently which might break some scripts users are running.

See GNU core utilities on Alpine wiki