feat: move from two-track to three-track container build

[wdconinc]

Briefly, what does this PR introduce?

This PR changes slightly the build structure from a two-track (builder, runtime) build to more of a three-track build (builder default, builder custom, runtime).

This primarily allows for concretization of the builder custom phase before any installation is done. Since we've been having some failures at that stage, it's annoying to have to wait until the installation is far along. This will trigger those failures earlier.

[Copilot]

Pull request overview

This PR refactors the Dockerfile build structure from a two-track (builder, runtime) to a three-track build (builder concretization, builder installation, runtime). The key benefit is that custom version concretization now happens earlier in the build process, allowing concretization failures to surface before expensive installation steps are executed.

Key Changes

• Modified builder_concretization_custom to derive from builder_concretization_default instead of builder_installation_default, enabling earlier failure detection
• Consolidated runtime stages by merging runtime_concretization_* and runtime_installation_* into single runtime_default and runtime_custom stages
• Updated documentation and ASCII diagram to reflect the new three-track architecture

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[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated 2 comments.

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

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[Copilot]

@wdconinc I've opened a new pull request, #117, to work on those changes. Once the pull request is ready, I'll request review from you.