Merge remote-tracking branch 'remotes/stsquad/tags/pull-docker-fixes-for-3.0-240718-1' into staging

docker fixes & tcg test tweak

  - graceful handling of testing under cross-compile
  - fixes for debootstrap handling
  - more helpful errors (binfmt_misc/EXECUTABLE missing)
  - drop runcom TCG test

# gpg: Signature made Tue 24 Jul 2018 11:48:32 BST
# gpg:                using RSA key FBD0DB095A9E2A44
# gpg: Good signature from "Alex Bennée (Master Work Key) <alex.bennee@linaro.org>"
# Primary key fingerprint: 6685 AE99 E751 67BC AFC8  DF35 FBD0 DB09 5A9E 2A44

* remotes/stsquad/tags/pull-docker-fixes-for-3.0-240718-1:
  tests/tcg: remove runcom test
  docker: perform basic binfmt_misc validation in docker.py
  docker: ignore distro versioning of debootstrap
  docker: add commentary to debian-bootstrap.docker
  docker: Update debootstrap script after Debian migration from Alioth to Salsa
  docker: report hint when docker.py check fails
  docker: drop QEMU_TARGET check, fallback in EXECUTABLE not set
  docker: add expansion for docker-test-FOO to Makefile.include
  docker: add test-unit runner
  docker: Makefile.include don't include partial images
  docker: gracefully skip check_qemu
  docker: move make check into check_qemu helper
  docker: split configure_qemu from build_qemu
  docker: fail more gracefully on docker.py check
  docker: par down QEMU_CONFIGURE_OPTS in debian-tricore-cross
  docker: base debian-tricore on qemu:debian9
  tests/.gitignore: don't ignore docker tests

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>