docs/devel: expand on use of containers to build tests

Expand on the usage of containers for building tests and why we have
some that are not used to build QEMU itself.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Cc: Thomas Huth <thuth@redhat.com>
Message-Id: <20210222101455.12640-5-alex.bennee@linaro.org>

docs/devel/testing.rst

@@ -272,10 +272,10 @@ Note that the following group names have a special meaning:
 
 - disabled: Tests in this group are disabled and ignored by check.
 
-.. _docker-ref:
+.. _container-ref:
 
-Docker based tests
-==================
+Container based tests
+=====================
 
 Introduction
 ------------
@@ -1001,10 +1001,17 @@ for the architecture in question, for example::
 There is also a ``--cross-cc-flags-ARCH`` flag in case additional
 compiler flags are needed to build for a given target.
 
-If you have the ability to run containers as the user you can also
-take advantage of the build systems "Docker" support. It will then use
-containers to build any test case for an enabled guest where there is
-no system compiler available. See :ref:`docker-ref` for details.
+If you have the ability to run containers as the user the build system
+will automatically use them where no system compiler is available. For
+architectures where we also support building QEMU we will generally
+use the same container to build tests. However there are a number of
+additional containers defined that have a minimal cross-build
+environment that is only suitable for building test cases. Sometimes
+we may use a bleeding edge distribution for compiler features needed
+for test cases that aren't yet in the LTS distros we support for QEMU
+itself.
+
+See :ref:`container-ref` for more details.
 
 Running subset of tests
 -----------------------