.dockerignore file is similar to .gitignore and serves the purpose to
simply ignore paths in the build context.
For now, let me just use it to fix the following problem:
```
docker build --build-arg KATA_ARTIFACTS=kata-static.tar.xz .
error checking context: 'no permission to read from
'(...)/local-build/build/firecracker/builddir/firecracker/(...)/crc64-1.0.0/.gitignore''.
```
Fixes: #2845
Signed-off-by: Fabiano Fidêncio <fabiano@fidencio.org>
