ci(reusable_build_image): Add support for nested boil images

[NickLarsenNZ]

Needed so that boil can find the config via the 'product-name', while using the 'namespace' and 'image-name' (without the parent directory) to tag the image with before pushing.

Details

The build action uses the product-name to locate the boil config. Eg:

product-name: hadoop # reads hadoop/boil-config.toml
namespace: sdp # this is used when the image is tagged

This broke twice when I wanted to build precompiled/hadoop:

product-name: hadoop
namespace: precompiled # want to put it in a different project in Harbor

That built the product image via hadoop/boil-config.toml.

So I changed it to:

product-name: precompiled/hadoop # Now it builds the hadoop source
namespace: precompiled # but now we have a duplicate "precompiled" when tagging the image

Waiting on two workflow runs (to ensure it fixes the issue and doesn't break existing stuff):

• 🟢 precompiled/hadoop (nested)
  • Correctly builds and publishes oci.stackable.tech/precompiled/hadoop:3.4.2-stackable1776715076
• 🟢 vector (not nested, and relatively fast)
  • Correctly builds and publishes oci.stackable.tech/sdp/vector:0.52.0-stackable0.0.0-dev

[soenkeliebau]

As discussed in Slack, discussed with and approved by Sascha. Approval for expediency