build: fix docker container provenance generation using json matrix strategy

[rsoberano-ld]

Requirements

• I have added test coverage for new or changed functionality
• I have followed the repository's pull request submission guidelines
• I have validated my changes against all supported platform versions

Related issues

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Describe the solution you've provided

The SLSA generator for docker container only accepts one image/digest combination at a time, whereas goreleaser may publish multiple image/digests in one release.

The goal here is to have the goreleaser step output a JSON array with image and digest properties for each image published, like so:

[{"image":"launchdarkly/ld-relay","digest":"sha256:abbeec99b023cd040fa89b3c30335fd123680a20a6abdff023eb1f23d65217e9"},{"image":"launchdarkly/ld-relay","digest":"sha256:abbeec99b023cd040fa89b3c30335fd123680a20a6abdff023eb1f23d65217e9"},{"image":"launchdarkly/ld-relay","digest":"sha256:abbeec99b023cd040fa89b3c30335fd123680a20a6abdff023eb1f23d65217e9"},{"image":"launchdarkly/ld-relay","digest":"sha256:abbeec99b023cd040fa89b3c30335fd123680a20a6abdff023eb1f23d65217e9"},{"image":"launchdarkly/ld-relay","digest":"sha256:abbeec99b023cd040fa89b3c30335fd123680a20a6abdff023eb1f23d65217e9"},{"image":"launchdarkly/ld-relay","digest":"sha256:abbeec99b023cd040fa89b3c30335fd123680a20a6abdff023eb1f23d65217e9"},{"image":"launchdarkly/ld-relay","digest":"sha256:ae50b3993d45ffcec26a602abbec7d4fc6f0859d7efaf8786d547d7e9f2fba47"},{"image":"launchdarkly/ld-relay","digest":"sha256:ae50b3993d45ffcec26a602abbec7d4fc6f0859d7efaf8786d547d7e9f2fba47"},{"image":"launchdarkly/ld-relay","digest":"sha256:ae50b3993d45ffcec26a602abbec7d4fc6f0859d7efaf8786d547d7e9f2fba47"},{"image":"launchdarkly/ld-relay","digest":"sha256:c6f2c654806e8adad6e0f98ab326517b02ce13e6ffb385e6d2537dade1be13c4"},{"image":"launchdarkly/ld-relay","digest":"sha256:c6f2c654806e8adad6e0f98ab326517b02ce13e6ffb385e6d2537dade1be13c4"},{"image":"launchdarkly/ld-relay","digest":"sha256:c6f2c654806e8adad6e0f98ab326517b02ce13e6ffb385e6d2537dade1be13c4"}]

Then we pass this JSON array as a matrix strategy for the provenance generator, with the idea that the provenance generator should run once per image/digest combo in the array.

Github actions are super finicky with formatting and JSON handling though, so I'm not 100% sure if this will work as we have it here - need to be able to test this somehow.

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Additional context

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[cwaldren-ld]

https://github.com/launchdarkly/ld-relay/actions/runs/9669190375