docker: add special handling for FROM:debian-%-user targets

These will have been build with debootstrap so we need to check
against the debian-bootstrap dockerfile. This does mean sticking to
debian-FOO-user as the naming conventions for boot-strapped images.
The actual cross image is built on top.
Signed-off-by: NAlex Bennée <alex.bennee@linaro.org>
Reviewed-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>

tests/docker/docker.py

@@ -113,6 +113,10 @@ def _copy_binary_with_libs(src, dest_dir):
             _copy_with_mkdir(l , dest_dir, so_path)
 
 def _read_qemu_dockerfile(img_name):
+    # special case for Debian linux-user images
+    if img_name.startswith("debian") and img_name.endswith("user"):
+        img_name = "debian-bootstrap"
+
     df = os.path.join(os.path.dirname(__file__), "dockerfiles",
                       img_name + ".docker")
     return open(df, "r").read()