ci: reuse trusted cache for Daytona images (#12862)

## Thinking Path

> - Paperclip is the open source app people use to manage AI agents for
work.
> - The full-stack runner campaign checks local and Daytona runner
behavior.
> - A Daytona image content miss starts a cold multi-stage Docker build.
> - Stable dependency and agent CLI layers take most of the image build
time.
> - Development targets must not write shared cache state.
> - This pull request adds a registry cache with a default-branch write
gate.
> - It also puts volatile source inputs after stable install layers.
> - The benefit is a shorter Daytona image build without weaker secret
isolation.

## Linked Issues or Issue Description

**What existing behavior does this improve?**

This improves the Daytona runner image stage in the full-stack E2E
workflow.

**Subsystem affected**

The GitHub Actions runner E2E workflow and its Daytona Docker image are
affected.

**Current behavior**

Each new Daytona image content ID starts with an empty BuildKit cache. A
runner source change also invalidates dependency and agent CLI install
layers because volatile inputs occur before those layers.

**Proposed behavior**

All authorized campaigns can read one GHCR BuildKit cache. Only a
campaign whose target ref is the repository default branch can update
that cache. The Dockerfile installs dependencies and agent CLIs before
it consumes volatile runner source or revision metadata.

**Reason and benefit**

The paid runner matrix spends several minutes building the image before
any selected cell can start. Cache reuse removes repeated stable setup
work and makes focused Daytona iterations faster.

**Breaking changes**

None. The immutable content tag, digest inspection, Cosign signature,
image labels, pinned base images, and provider credential boundary stay
unchanged.

## What Changed

- Read a registry-backed BuildKit cache for Daytona image content
misses.
- Export the cache only when the resolved target ref is the default
branch.
- Keep provider credentials outside the image build and cache.
- Install provider-pack dependencies before runner source is copied.
- Keep expensive agent CLI installs before source revision metadata.
- Add workflow and Docker layer-order contract checks.

## Verification

- `prettier --write .github/workflows/runner-full-stack-e2e.yml
tests/runner-e2e/daytona-image.test.ts
tests/runner-e2e/workflow-security.test.ts`
- `actionlint .github/workflows/runner-full-stack-e2e.yml`
- `git diff --check`
- I did not run a test suite or Docker image build locally. The
requested iteration policy reserves those checks for GitHub Actions.

## Risks

Low risk. BuildKit can use a cache record only when its content key
matches the build instruction and input. Development targets have
read-only cache access. The cache contains public source and build
outputs, but it does not receive provider credentials or the GitHub
token as Docker build inputs.

> For core feature work, check [`ROADMAP.md`](ROADMAP.md) first and
discuss it in `#dev` before opening the PR. Feature PRs that overlap
with planned core work may need to be redirected — check the roadmap
first. See `CONTRIBUTING.md`.

## Model Used

OpenAI Codex with GPT-5, tool use, and code execution.

## Checklist

- [x] I have included a thinking path that traces from project context
to this change
- [x] I have specified the model used (with version and capability
details)
- [x] I have checked ROADMAP.md and confirmed this PR does not duplicate
planned core work
- [x] I have searched GitHub for duplicate or related PRs and linked
them above
- [x] I have either (a) linked existing issues with `Fixes: #` / `Closes
#` / `Refs #` OR (b) described the issue in-PR following the relevant
issue template
- [x] I have not referenced internal/instance-local Paperclip issues or
links (only public GitHub `#NNN` / `github.com/paperclipai/paperclip`
URLs)
- [x] My branch name describes the change (e.g. `docs/...`, `fix/...`)
and contains no internal Paperclip ticket id or instance-derived details
- [ ] I have run tests locally and they pass
- [x] I have added or updated tests where applicable
- [x] I have updated relevant documentation to reflect my changes
- [x] I have considered and documented any risks above
- [ ] All Paperclip CI gates are green
- [ ] Greptile is 5/5 with no open P2s, recommendations, or follow-ups
- [x] I will address all Greptile and reviewer comments before
requesting merge

docker/daytona-runner/Dockerfile

@@ -13,24 +13,17 @@ RUN cargo build --locked --release -p paperclip-runner-core --bin paperclip-runn
     && strip /workspace/packages/paperclip-runner/runner/target/release/paperclip-runnerd
 
 FROM node:24-bookworm@sha256:9137a20e25879e0b557227b57e3ee4e9af4bde29eb3db66134cd1723e84f830b AS provider-pack-build
-ARG PAPERCLIP_RUNNER_SOURCE_REVISION
-RUN test -n "${PAPERCLIP_RUNNER_SOURCE_REVISION}"
 RUN corepack enable && corepack prepare pnpm@9.15.4 --activate
 WORKDIR /workspace
 COPY package.json pnpm-lock.yaml pnpm-workspace.yaml .npmrc tsconfig.base.json ./
 COPY patches ./patches
 COPY scripts/link-plugin-dev-sdk.mjs ./scripts/link-plugin-dev-sdk.mjs
-COPY \
-    packages/paperclip-eval-kernel/package.json \
-    packages/paperclip-eval-kernel/tsconfig.json \
-    ./packages/paperclip-eval-kernel/
+COPY packages/paperclip-eval-kernel/package.json ./packages/paperclip-eval-kernel/package.json
+COPY packages/paperclip-runner/package.json ./packages/paperclip-runner/package.json
+RUN pnpm install --frozen-lockfile --filter '@paperclipai/paperclip-runner...'
+COPY packages/paperclip-eval-kernel/tsconfig.json ./packages/paperclip-eval-kernel/tsconfig.json
 COPY packages/paperclip-eval-kernel/src ./packages/paperclip-eval-kernel/src
-COPY \
-    packages/paperclip-runner/package.json \
-    packages/paperclip-runner/styles.css \
-    packages/paperclip-runner/tsconfig.json \
-    packages/paperclip-runner/tsconfig.surfaces.json \
-    ./packages/paperclip-runner/
+COPY packages/paperclip-runner/styles.css packages/paperclip-runner/tsconfig.json packages/paperclip-runner/tsconfig.surfaces.json ./packages/paperclip-runner/
 COPY packages/paperclip-runner/protocol ./packages/paperclip-runner/protocol
 COPY packages/paperclip-runner/runner/crates/runner-core/src/generated_acpx_sidecar_contract.rs ./packages/paperclip-runner/runner/crates/runner-core/src/generated_acpx_sidecar_contract.rs
 COPY \
@@ -41,7 +34,8 @@ COPY \
     packages/paperclip-runner/scripts/generate-protocol-schema-module.mjs \
     ./packages/paperclip-runner/scripts/
 COPY packages/paperclip-runner/src ./packages/paperclip-runner/src
-RUN pnpm install --frozen-lockfile --filter '@paperclipai/paperclip-runner...'
+ARG PAPERCLIP_RUNNER_SOURCE_REVISION
+RUN test -n "${PAPERCLIP_RUNNER_SOURCE_REVISION}"
 RUN pnpm --filter @paperclipai/paperclip-runner build:typescript \
     && PAPERCLIP_RUNNER_SOURCE_REVISION="${PAPERCLIP_RUNNER_SOURCE_REVISION}" \
       node packages/paperclip-runner/scripts/build-provider-pack.mjs /provider-pack
@@ -51,13 +45,6 @@ RUN pnpm --filter @paperclipai/paperclip-runner build:typescript \
 # runner-specific addition is /usr/local/bin/paperclip-runnerd below.
 FROM daytonaio/sandbox:0.8.0@sha256:eadf88e4391072b7ad4bed27d9cadfc9fe9d8ed375d9219d34c2ccb518f213e3
 
-ARG PAPERCLIP_RUNNER_CONTENT_ID
-ARG PAPERCLIP_RUNNER_SOURCE_REVISION
-RUN test -n "${PAPERCLIP_RUNNER_CONTENT_ID}" \
-    && test -n "${PAPERCLIP_RUNNER_SOURCE_REVISION}"
-LABEL io.paperclip.runner.content-id="${PAPERCLIP_RUNNER_CONTENT_ID}" \
-      org.opencontainers.image.revision="${PAPERCLIP_RUNNER_SOURCE_REVISION}"
-
 USER root
 
 ENV PAPERCLIP_RUNNER_PROVIDER_PACK_ROOT=/opt/paperclip-runner/provider-pack
@@ -104,6 +91,14 @@ RUN set -eu; \
 COPY --from=runnerd-build /workspace/packages/paperclip-runner/runner/target/release/paperclip-runnerd /usr/local/bin/paperclip-runnerd
 COPY --from=provider-pack-build /provider-pack /opt/paperclip-runner/provider-pack
 
+# Keep revision-dependent metadata below the stable agent CLI installation
+# layers. A source-only image miss can then reuse those expensive layers from
+# the trusted registry cache.
+ARG PAPERCLIP_RUNNER_CONTENT_ID
+ARG PAPERCLIP_RUNNER_SOURCE_REVISION
+RUN test -n "${PAPERCLIP_RUNNER_CONTENT_ID}" \
+    && test -n "${PAPERCLIP_RUNNER_SOURCE_REVISION}"
+
 RUN set -eu; \
     chmod -R a+rX /opt/paperclip-runner/provider-pack; \
     printf '%s\n' 'export PATH=/opt/paperclip-runner/provider-pack/node_modules/.bin:$PATH' \
@@ -132,3 +127,6 @@ RUN /bin/sh -lc 'set -eu; \
     test "$(acpx --version)" = "0.13.1"; \
     test "$(claude-agent-acp --version)" = "0.70.0"; \
     test "$(codex-acp --version)" = "@agentclientprotocol/codex-acp 1.6.2"'
+
+LABEL io.paperclip.runner.content-id="${PAPERCLIP_RUNNER_CONTENT_ID}" \
+      org.opencontainers.image.revision="${PAPERCLIP_RUNNER_SOURCE_REVISION}"