I'm getting this error with cache mounts:

"dockerfile invalid: flag '--mount=type=cache,id=s/...' is missing the cacheKey prefix from its id"

According to docs (https://docs.railway.com/guides/dockerfiles):

--mount=type=cache,id=s/-,target=

WHAT I TRIED (ALL FAILED):

1. Exact doc format with full path:

--mount=type=cache,id=s/f73581c6-c2d7-432a-803d-136b57ce3ce6-/root/.bun/install/cache,target=/root/.bun/install/cache

Error: missing cacheKey prefix

2. Simple name (from community examples):

--mount=type=cache,id=s/f73581c6-c2d7-432a-803d-136b57ce3ce6-bun-cache,target=/root/.bun/install/cache

Error: missing cacheKey prefix

3. Without s/ prefix:

--mount=type=cache,id=f73581c6-c2d7-432a-803d-136b57ce3ce6-bun-cache,target=/root/.bun/install/cache

Error: Cache mount ID is not prefixed with cache key

4. Short name:

--mount=type=cache,id=s/f73581c6-c2d7-432a-803d-136b57ce3ce6-bun,target=/root/.bun/install/cache

Error: missing cacheKey prefix

QUESTIONS:

- Is there a hidden "cacheKey" prefix needed?

- Has the format changed recently?

- Can you provide a working example with real service ID?

ENVIRONMENT:

- Base: oven/bun:1-alpine

- BuildKit: enabled (Railway default)

- Service ID: f73581c6-c2d7-432a-803d-136b57ce3ce6

Every format fails with the same error. Is this feature actually working?

WORKAROUND:

Removed cache mounts, using Docker layer caching only.