Interim Bugfix Release - Bazel 4.0.0 compatible

This is just an interim release of bazelbuild/rules_docker containing Bazel 4.0.0 compatibility fixes

Copy the following into your WORKSPACE file to use rules_docker at release v0.15.1-alpha

http_archive(
    name = "io_bazel_rules_docker",
    sha256 = "a139e494d955fa133acb48bd7adc1a0b803139c0649f690c60b711700a24ec30",
    strip_prefix = "rules_docker-0.15.1-alpha",
    urls = ["https://github.com/dmayle/rules_docker/releases/download/v0.15.1-alpha/rules_docker-v0.15.1-alpha.tar.gz"],
)

Changes:

1. Add option to push images sequentially in push-all (bazelbuild#1658)
2. Added s390x support to load puller and loader binary (bazelbuild#1661)
3. Fix update_deps to grab correct platform digest, rerun (bazelbuild#1666)
4. Add arm64 support to load puller and loader binaries (bazelbuild#1671)
5. Fix invalid escape sequence (bazelbuild#1675)
6. Fix more invalid escape sequences (bazelbuild#1676)
7. Relax error string matching (bazelbuild#1677)
8. Fix conditional select on darwin. (bazelbuild#1674)
9. Fix Stamp param propagation from base image (bazelbuild#1672)
10. Improve portability of download_pkgs genrule bash (bazelbuild#1673)
11. Add experimental_tarball_format to container_image (bazelbuild#1696)
12. Introduce output_group for pusher executable (bazelbuild#1685)
13. Fix broken tests resulting from pip dropping support for legacy Python versions. (bazelbuild#1723)
14. Support computing the digest when caller can only specify --tarball. (bazelbuild#1721)
15. Renamed io_bazel_rules_rust to rules_rust (bazelbuild#1720)
16. Select thhe structure_test_executable for the execution platform (bazelbuild#1727)