When you use Docker’s new BuildKit build engine either by
DOCKER_BUILDKIT=1 docker build ...
or in most recent Docker v19 using the new buildx command
docker buildx build ...
then you won’t see any output from your RUN steps. For example the following Dockerfile
FROM alpine
RUN echo Hello
then you won’t see a ‘Hello’ in the output (produced with docker version 18.09.7):
$
$ DOCKER_BUILDKIT=1 docker build . --no-cache
[+] Building 0.7s (6/6) FINISHED
=> [internal] load build definition from Dockerfile 0.0s
=> => transferring dockerfile: 37B 0.0s
=> [internal] load .dockerignore 0.0s
=> => transferring context: 2B 0.0s
=> [internal] load metadata for docker.io/library/alpine:latest 0.0s
=> CACHED [1/2] FROM docker.io/library/alpine 0.0s
=> [2/2] RUN echo Hello 0.6s
=> exporting to image 0.0s
=> => exporting layers 0.0s
=> => writing image sha256:ee9833c6e7bfa7e64fa74b1aa5da5a1818dc881929651 0.0s
$
So while it tells about the RUN step, it doesn’t give you the output.
Now with BuildKit comes a new option for docker build named --progress that let’s you switch between three output modes:
- auto (default)
- plain
- tty
Interestingly there are sources that say ‘tty’ is supposed to be the legacy output. But at least for me it does give no different output then ‘auto’. I guess because ‘auto’ selects ‘tty’ and ‘tty’ is what it is…
So this leave ‘plain’ which produced a very interesting block output, which actually lists the RUN command result:
$ DOCKER_BUILDKIT=1 docker build --progress=plain .
[...]
#5 [2/2] RUN echo Hello
#5 digest: sha256:a3caaceea4f05ba90a9dea76e435897fe6454097eb90233e58530b6e1e7f7a53
#5 name: "[2/2] RUN echo Hello"
#5 started: 2020-03-13 18:52:22.20543247 +0000 UTC
#5 0.496 Hello
#5 completed: 2020-03-13 18:52:22.750641665 +0000 UTC
#5 duration: 545.209195ms
[...]
One can argue about usability of this, still this is the way to go to get command output when using BuildKit.