[PATCH v3 0/6] Travis + Azure jobs for linux with musl libc

Recently, we've un-broken git for Linux with musl libc, and we have a serie to fix false negative with busybox shell utils.

Add a CI job on Travis and Azure to make sure we won't break it again.

This is a nearly rewrite of this series, because there're GitHub Action allow running directly inside container.

=> I rewrite this series to prepare as much as possible for the GitHub Action series. => No range-diff

The first patch is coming from Szeder, Junio hasn't picked it up yet. And, this series depends on it.

Sample build without busybox fix series: https://travis-ci.org/github/sgn/git/builds/670097222

With busybox fix: https://travis-ci.org/github/sgn/git/builds/670103249

SZEDER Gábor (1):
  ci: make MAKEFLAGS available inside the Docker container in the
    Linux32 job

Đoàn Trần Công Danh (5):
  ci/lib-docker: preserve required environment variables
  ci/linux32: parameterise command to switch arch
  ci: refactor docker runner script
  ci/linux32: libify install-dependencies step
  travis: build and test on Linux with musl libc and busybox

 .travis.yml                                   | 10 ++++-
 azure-pipelines.yml                           | 39 ++++++++++++++++++-
 ci/install-docker-dependencies.sh             | 18 +++++++++
 ci/lib.sh                                     |  8 ++++
 ...n-linux32-build.sh => run-docker-build.sh} | 39 +++++++++++++------
 ci/{run-linux32-docker.sh => run-docker.sh}   | 28 ++++++++++---
 6 files changed, 121 insertions(+), 21 deletions(-)
 create mode 100755 ci/install-docker-dependencies.sh
 rename ci/{run-linux32-build.sh => run-docker-build.sh} (63%)
 rename ci/{run-linux32-docker.sh => run-docker.sh} (43%)