>>> b 4
>>> bp
[0]: line: Dockerfile:4
[on-fail]: breaks on fail
>>> c
Breakpoint[0]: reached line: Dockerfile:4
Breakpoint[on-fail]: caught error process "/bin/sh -c echo -n c > /" did not complete successfully: exit code: 1
Filename: "Dockerfile"
      1| FROM registry2-buildkit:5000/busybox:1
      2| RUN echo -n a > /a
      3| RUN echo -n b > /a
*=>   4| RUN echo -n c > /
      5| RUN echo -n d > /d