Properly detect Alpine virtual packages

[pombredanne]

Description

Alpine linux installed packages may be "virtual" such as with this snippet of installed DB

P:.postgis-rundeps
V:20210104.190748
A:noarch
S:0
I:0
T:virtual meta package
U:
L:
D:json-c geos gdal proj protobuf-c libstdc++

This was created from this https://github.com/postgis/docker-postgis/blob/e5e0e9b3b38acb6e4f1f16ed0c697c9f6cb2cf1e/Dockerfile.alpine.template#L61

.postgis-rundeps was created at install time and does not exist as a "real" package. This is instead just a collection of dependencies, therefore it cannot have much of an origin or license. This is a "virtual meta package". It can be identified because of its installed "description" with this value "T:virtual meta package"

[pombredanne]

Other examples:

• https://github.com/docker-library/redis/blob/768afb5842d4d8daeab875d886771922749a0db3/Dockerfile-alpine.template#L90
• https://github.com/docker-library/python/blob/2bf869e357137d4d6d5c18afbfda284848cd46d1/Dockerfile-alpine.template#L136