Pin parent image of `Dockerfile` to `debian:bullseye-20210511`.
Pin parent image of `Dockerfile` to `debian:bullseye-20210511`.

This avoids changes from `debian:bullseye-20210621` whose updates
created breakages with the LLVM bazel toolchain (see #10).

GitOrigin-RevId: 8e7f109ce7d8cb9b8a6a5374257470901a3bb02b
Change-Id: I93cc4f463245201c526bf24724156298f8e133fc

google/fully-homomorphic-encryption,google/fully-homomorphic-encryption