Fix Dockerfile mount syntax for Railway builder

[railway-app]

Problem

The DOCKERFILE builder was failing at BUILD_IMAGE with 6 validation errors. The ext-deps stage used --mount=type=bind which Railway does not support (only type=cache is allowed), and all five --mount=type=cache instructions used custom IDs like openclaw-pnpm-store and openclaw-bookworm-apt-cache instead of the required Railway format id=s/<service-id>-<target-path>.

Solution

Replaced the --mount=type=bind in the ext-deps stage with a COPY instruction followed by a plain RUN, which achieves the same result without a bind mount. Updated all five cache mount IDs to the Railway-required format id=s/a1749301-c61b-4abd-9a55-b8572c7232f1-<target-path> — one for the pnpm store and four for the apt cache/lists mounts across the runtime stage.

Changes

• Modified Dockerfile

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