unify apk installed db for base layer

[deitch]

Signed-off-by: Avi Deitcher avi@deitcher.net

- What I did

Fixed an issue wherein multiple layers in init trounce each other's apk installed db file.

Specifically, let's say I have some images:

init:
  - linuxkit/init:8f1e6a0747acbbb4d7e24dc98f97faa8d1c6cec7
  - linuxkit/runc:f01b88c7033180d50ae43562d72707c6881904e4
  - linuxkit/containerd:de1b18eed76a266baa3092e5c154c84f595e56da
  - linuxkit/ca-certificates:c1c73ef590dffb6a0138cf758fe4a4305c9864f4

Each of the above gets laid out, in order, on the root filesystem.

If all of them have apk packages installed, then all of them have /lib/apk/db/installed. However, as this is just a tar stream, the last one (in the above example, from linuxkit/ca-certificates) overwrites the previous. That means that the installed db is out of date.

This PR changes it so that we capture all of the installed files and concatenate them to have a single one that includes all of them.

Right now, it just concatenates, so it likely has duplicates, but it still works. If I can get deduplication before this goes in, I will comment here.

- How I did it

Added a tar wrapper for when the init sections are streamed, so it captures the /lib/apk/db/installed databases and then merges them when told to.

- How to verify it

CI, plus a manual test to see that it is merged.

- Description for the changelog

init captures all apk installed databases

[deitch]

I can confirm this works. The tar stream might be smaller if we had only the final version, but that can be a later optimization.