Add .cog to a .dockerignore produces hard to debug states

[8W9aG]

If you add .cog to a .dockerignore it produces issues like the following:

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 > [stage-0 3/7] COPY .cog/tmp/build20240919160513.8228582876843814/requirements.txt /tmp/requirements.txt:
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Dockerfile:4
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   2 |     FROM r8.im/cog-base:cuda11.8-python3.11
   3 |     RUN --mount=type=cache,target=/var/cache/apt,sharing=locked apt-get update -qq && apt-get install -qqy  && rm -rf /var/lib/apt/lists/*
   4 | >>> COPY .cog/tmp/build20240919160513.8228582876843814/requirements.txt /tmp/requirements.txt
   5 |     ENV CFLAGS="-O3 -funroll-loops -fno-strict-aliasing -flto -S"
   6 |     RUN pip install --no-cache-dir -r /tmp/requirements.txt
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ERROR: failed to solve: failed to compute cache key: failed to calculate checksum of ref 50e2db01-7591-4a90-a5ab-e68b1ae19a0e::qc781cqwiy4yk3g558ya3mubm: "/.cog/tmp/build20240919160513.8228582876843814/requirements.txt": not found

This is quite difficult to debug, we should either warn the user that adding .cog produces undesireable effects or filter this out of the .dockerignore file produced by the user.

@zsxkib