How to debug when docker build fails

Introduction#

When a docker build -t mytag . fails with a message such as

---> Running in d9a42e53eb5a

The command '/bin/sh -c returned a non-zero code: 127

(127 means “command not found, but

1. it is not trivial for everybody
2. 127 may be replaced by 6 or anything)

it may be non trivial to find the error in a long line

basic example

As the last layer created by

docker build -t mytag .

showed

---> Running in d9a42e53eb5a

You just launch the last created image with a shell and launch the command, and you will have a more clear error message

docker run -it d9a42e53eb5a /bin/bash

(this assumes /bin/bash is available, it may be /bin/sh or anything else)

and with the prompt, you launch the last failing command, and see what is displayed