Fix typo in Dockerfile

[symkat]

I believe this is a typo, when trying fly launch --from https://github.com/fly-apps/user-env-api --yes I would get the following errors:

 => ERROR [stage-1 4/7] COPY --from=builder /app/bun.lockb ./                                                                                                    0.0s
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 > [stage-1 4/7] COPY --from=builder /app/bun.lockb ./:
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Error: failed to fetch an image or build from source: error building: failed to solve: failed to compute cache key: failed to calculate checksum of ref p319nmowb4ddkluoaqavdwzs0::5kezq3s8408i1616u0yz993zm: "/app/bun.lockb": not found

I updated the Dockerfile locally with this change and then fly deploy worked.