Why Docker COPY doesn't change file permissions? (--chmod) - linux

Question

Given this Dockerfile:
FROM docker.io/alpine
RUN mkdir test
# RUN umask 0022
COPY README /test/README
COPY --chmod=777 README /test/README-777
COPY --chmod=755 README /test/README-755
COPY FORALL /test/FORALL
COPY --chmod=777 FORALL /test/FORALL-777
COPY --chmod=755 FORALL /test/FORALL-755
RUN ls -la /test
I'd expect to have the read, write, execute permissions be set accordingly by Docker during the build process (docker build ./).
But the last command returns
total 8
drwxr-xr-x 1 root root 4096 Jun 9 19:20 .
drwxr-xr-x 1 root root 4096 Jun 9 19:20 ..
-rwxrwxrwx 1 root root 0 Jun 9 19:19 FORALL
-rwxrwxrwx 1 root root 0 Jun 9 19:19 FORALL-755
-rwxrwxrwx 1 root root 0 Jun 9 19:19 FORALL-777
-rw-rw-r-- 1 root root 0 Jun 9 19:19 README
-rw-rw-r-- 1 root root 0 Jun 9 19:19 README-755
-rw-rw-r-- 1 root root 0 Jun 9 19:19 README-777
No file permission was changed, and no error was raised.
Why doesn't it work?
How to fix this?

Answer 1

I figured out:
the flag --chmod is a new feature from Docker Buildkit, so it is necessary to run the build enabling it via:
DOCKER_BUILDKIT=1 docker build ./
However, it is really not clear why Docker swallows the --chmod option without any error or warn about the non-existing option 😕.

Answer 2

This is fixed in 20.10.6 (pull request, tracking issue):
$ cat df.chmod
FROM busybox as base
RUN touch /test
FROM busybox as release
COPY --from=base --chmod=777 /test /test-777
COPY --from=base --chmod=555 /test /test-555
CMD ls -l /test*
$ DOCKER_BUILDKIT=0 docker build -t test-chmod-classic -f df.chmod .
Sending build context to Docker daemon 22.02kB
Step 1/6 : FROM busybox as base
---> a9d583973f65
Step 2/6 : RUN touch /test
---> Running in ed48f45a5dca
Removing intermediate container ed48f45a5dca
---> 5606d2d23861
Step 3/6 : FROM busybox as release
---> a9d583973f65
Step 4/6 : COPY --from=base --chmod=777 /test /test-777
the --chmod option requires BuildKit. Refer to https://docs.docker.com/go/buildkit/ to learn how to build images with BuildKit enabled
And if the build is run with buildkit, the expected result occurs:
$ DOCKER_BUILDKIT=1 docker build -t test-chmod-buildkit -f df.chmod .
[+] Building 1.0s (8/8) FINISHED
=> [internal] load build definition from df.chmod 0.0s
=> => transferring dockerfile: 214B 0.0s
=> [internal] load .dockerignore 0.0s
=> => transferring context: 49B 0.0s
=> [internal] load metadata for docker.io/library/busybox:latest 0.0s
=> CACHED [base 1/2] FROM docker.io/library/busybox 0.0s
=> [base 2/2] RUN touch /test 0.6s
=> [release 2/3] COPY --from=base --chmod=777 /test /test-777 0.1s
=> [release 3/3] COPY --from=base --chmod=555 /test /test-555 0.1s
=> exporting to image 0.0s
=> => exporting layers 0.0s
=> => writing image sha256:a4df92175046e36a72a769f9c7b297bc04a825708c5f6ca5873428b55c340036 0.0s
=> => naming to docker.io/library/test-chmod-buildkit 0.0s
$ docker run --rm test-chmod-buildkit
-r-xr-xr-x 1 root root 0 Jun 10 13:00 /test-555
-rwxrwxrwx 1 root root 0 Jun 10 13:00 /test-777