Add a docker container for libc premerge tests

[amykhuang]

Currently the libc premerge fullbuild tests download a recent llvm as part of the build setup. This is prone to errors sometimes and it would be better to do this setup in a docker container.

Example of the Dockerfile used for the main llvm premerge setup is https://github.com/llvm/llvm-project/blob/main/.github/workflows/containers/github-action-ci/Dockerfile.

[llvmbot]

@llvm/issue-subscribers-libc

Author: Amy Huang (amykhuang)

Currently the libc premerge fullbuild tests download a recent llvm as part of the build setup. This is prone to errors sometimes and it would be better to do this setup in a docker container.

Example of the Dockerfile used for the main llvm premerge setup is https://github.com/llvm/llvm-project/blob/main/.github/workflows/containers/github-action-ci/Dockerfile.

[llvmbot]

@llvm/issue-subscribers-infrastructure

Author: Amy Huang (amykhuang)

Currently the libc premerge fullbuild tests download a recent llvm as part of the build setup. This is prone to errors sometimes and it would be better to do this setup in a docker container.

Example of the Dockerfile used for the main llvm premerge setup is https://github.com/llvm/llvm-project/blob/main/.github/workflows/containers/github-action-ci/Dockerfile.