Set layer media types consistently

[imjasonh]

We currently always append layers with Docker media types, even if the base image is an OCI image (e.g., our new default base image, ghcr.io/distroless/static)

This works fine against some registries (GCR), but not against others (apparently Quay, Gitlab Registry, probably others).

This was reported in tektoncd/pipeline#5188 since Tekton switched its base image for git-init to ghcr.io/distroless/git

This changes our layer creation to check for the type of the base image, and set layer media types accordingly.

[codecov-commenter]

Codecov Report

Merging #776 (ea7d482) into main (d32cfff) will increase coverage by 0.10%.
The diff coverage is 82.35%.

@@            Coverage Diff             @@
##             main     #776      +/-   ##
==========================================
+ Coverage   51.20%   51.30%   +0.10%     
==========================================
  Files          44       44              
  Lines        3316     3329      +13     
==========================================
+ Hits         1698     1708      +10     
- Misses       1401     1403       +2     
- Partials      217      218       +1     

Impacted Files	Coverage Δ
pkg/build/gobuild.go	59.13% <82.35%> (+0.29%)	⬆️