How do I run pstree on alpine linux?

Question

Can I see the process list as a tree on alpine linux? I tried using apk add but it gave an error about unsatisfiable constraints. The error isn't very helpful :-(

/ # apk search pstools
WARNING: Ignoring APKINDEX.84815163.tar.gz: No such file or directory
WARNING: Ignoring APKINDEX.24d64ab1.tar.gz: No such file or directory

/ # apk search ps
WARNING: Ignoring APKINDEX.84815163.tar.gz: No such file or directory
WARNING: Ignoring APKINDEX.24d64ab1.tar.gz: No such file or directory

/ # apk add pstree
WARNING: Ignoring APKINDEX.84815163.tar.gz: No such file or directory
WARNING: Ignoring APKINDEX.24d64ab1.tar.gz: No such file or directory
ERROR: unsatisfiable constraints:
  pstree (missing):
    required by: world[pstree]

Thank you for reading.

Answer 1

The following commands helped me:

/#     apk update 
fetch http://dl-cdn.alpinelinux.org/alpine/v3.6/main/x86_64/APKINDEX.tar.gz
fetch http://dl-cdn.alpinelinux.org/alpine/v3.6/community/x86_64/APKINDEX.tar.gz
v3.6.3-58-g8a2e635d73 [http://dl-cdn.alpinelinux.org/alpine/v3.6/main]
v3.6.3-57-gfd0de89d0b [http://dl-cdn.alpinelinux.org/alpine/v3.6/community]
OK: 8448 distinct packages available

/ #    apk add pstree
(1/1) Installing pstree (2.39-r0)
Executing busybox-1.26.2-r5.trigger
OK: 35 MiB in 27 packages