Dockerfile: chmod on root directory not working - docker

Question

I build a docker image based on following Dockerfile on Ubuntu:
FROM openjdk:8-jre-alpine
USER root
RUN echo XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX && ls -ald /
RUN chmod 777 /
RUN echo XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX && ls -ald /
ENTRYPOINT [ "sh", "-c", "echo test" ]
I'm expecting that the root path obtains the set permissions but building the docker image outputs following (consider the output of ls -ald /):
docker build . -f Dockerfile
Sending build context to Docker daemon 2.048kB
Step 1/6 : FROM openjdk:8-jre-alpine
---> b76bbdb2809f
Step 2/6 : USER root
---> Using cache
---> 18045a1e2d82
Step 3/6 : RUN echo XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX && ls -ald /
---> Running in 2309a8753729
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
drwxr-xr-x 1 root root 4096 Mar 19 13:50 /
Removing intermediate container 2309a8753729
---> 809221ec8f71
Step 4/6 : RUN chmod 777 /
---> Running in 81df09ec266c
Removing intermediate container 81df09ec266c
---> 9ea5e2282356
Step 5/6 : RUN echo XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX && ls -ald /
---> Running in ef91613577da
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
drwxr-xr-x 1 root root 4096 Mar 19 13:50 /
Removing intermediate container ef91613577da
---> cd7914160661
Step 6/6 : ENTRYPOINT [ "sh", "-c", "echo test" ]
---> Running in 3d724aca37fe
Removing intermediate container 3d724aca37fe
---> 143e46ec55a8
Successfully built 143e46ec55a8
How can I determine the permissions?
UPDATE: I have specific reasons why I'm temporarily forced to set these permissions on root folder: Unfortunately, I'm running a specific application within the container with another user than root and this application writes something directly into /. Currently, this isn't configurable.
If I do it on another folder under root, it works as expected:
...
Step 6/8 : RUN mkdir -p /mytest && chmod 777 /mytest
---> Running in 7aa3c7b288fd
Removing intermediate container 7aa3c7b288fd
---> 1717229e5ac0
Step 7/8 : RUN echo ZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZ && ls -ald /mytest
---> Running in 2238987e1dd6
ZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZ
drwxrwxrwx 2 root root 4096 Mar 19 14:42 /mytest
...
On execution of container:
drwxrwxrwx 2 root root 4096 Mar 19 14:42 mytest

Answer 1

To check the permission of your root folder bash inside your container, perform following opertations
docker exec -it container_id bash
cd /
ls -ald