docker:buildkit: ensure container build output is generated

Some newer distributions are moving to buildkit which is an improved
backend to docker that replaces the legacy builder. This new backend
builder has different rules on what it outputs to the console as it
builds an image.

To ensure we get the normal verbose output we're used to, set the
BUILDKIT_PROGRESS env variable.

There's also a parameter you can pass to docker build (--progress) but
it doesn't appear in the --help of older docker build commands so it's
best to just utilize the env variable option.

Change-Id: I520c0b933f920b05937e68a76fb7bd02ff889bc4
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>
diff --git a/build-setup.sh b/build-setup.sh
index 53af227..74bcb51 100755
--- a/build-setup.sh
+++ b/build-setup.sh
@@ -395,6 +395,10 @@
 # Give the Docker image a name based on the distro,tag,arch,and target
 img_name=${img_name:-openbmc/${distro}:${img_tag}-${target}-${ARCH}}
 
+# Ensure appropriate docker build output to see progress and identify
+# any issues
+export BUILDKIT_PROGRESS=plain
+
 # Build the Docker image
 docker build -t "${img_name}" - <<< "${Dockerfile}"