Show summary of build warnings.

[tonistiigi]

In addition to showing warning where it happens show a summary after build has completed.

> docker buildx build .
...
 1 warning found (use --debug to expand):
 - Empty continuation line found in: RUN date &&     sleep 4 &&     date && echo $GREETING

with --debug

...
 1 warning found:
 - Empty continuation line found in: RUN date &&     sleep 4 &&     date && echo $GREETING
Empty continuation lines will become errors in a future release
More info: https://github.com/moby/moby/pull/33719
Dockerfile:11
--------------------
   9 |         sleep 4 && \
  10 |     
  11 | >>>     date && echo $GREETING
  12 |     RUN true
  13 |      
--------------------

Also fix the issue with progressbar and logging both printing to terminal at same time. Now logs are buffered while the terminal is active.

Additional commit suppresses some useless debug logs from vendored components.