fix: keep enclave signing key out of image builds

[smtmfft]

Summary

• remove the committed enclave signing key from the repo and Docker build context
• sign SGX and SGXGETH artifacts with a BuildKit secret instead of copying the key into image layers
• support release builds from either ENCLAVE_KEY_PATH or GCP Secret Manager, while keeping disposable-key fallback for direct CI/local Docker builds without a stable key
• scrub the historical Gramine key from the SGX runtime base inside the main Dockerfile via an internal scratch-based runtime-base-clean stage, so no external base-clean image needs to be published first

This supersedes #695 with only the enclave-key build hardening changes. It intentionally does not include the stale gaiko pointer, chainspec, Cargo.lock, or unrelated branch changes from #695.

Verification

• bash -n script/publish-image.sh
• git diff --check
• docker buildx build --check -f Dockerfile .
• rg for private-key headers returned no matches

[Copilot]

Pull request overview

This PR hardens the SGX/TEE build pipeline by removing the enclave signing private key from the repository and image build context, and switching signing steps to use BuildKit secrets so the key is never written into Docker image layers.

Changes:

• Updated Dockerfile to sign SGX/SGXGETH artifacts using RUN --mount=type=secret and an optional public-key hash check, instead of copying a private key into the build context/layers.
• Updated script/publish-image.sh to support supplying the signing key via ENCLAVE_KEY_PATH or GCP Secret Manager and pass it to docker buildx as a BuildKit secret.
• Removed the committed docker/enclave-key.pem, adjusted .dockerignore/.gitignore, and added docker/Dockerfile.base-clean to create a scrubbed runtime base image.

Reviewed changes

Copilot reviewed 5 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
script/publish-image.sh	Adds BuildKit secret wiring and optional GCP Secret Manager retrieval for enclave signing key during image builds.
Dockerfile	Switches enclave signing to BuildKit secrets and parameterizes the SGX runtime base image to a scrubbed base-clean.
docker/enclave-key.pem	Removes the committed private signing key from the repository.
docker/Dockerfile.base-clean	Adds a “scrubbed” base image recipe to remove the historical key from the runtime base filesystem.
.gitignore	Ensures any local docker/enclave-key.pem is not accidentally committed.
.dockerignore	Removes the exception that previously forced docker/enclave-key.pem into the Docker build context.

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