Docker mount available in in-container shell, but not from Dockerfile-CodePudding

I have a strange situation where a shell launched from within the container is able to see a mounted volume, but any CMD cannot see that volume for some reason.

I have the following source tree:

[~/workspace/docker-test]$ tree
.
├── Dockerfile
└── source
    └── file.txt

1 directory, 2 files

where Dockerfile is

FROM ubuntu:jammy as prereqs
RUN ls /usr/source

When I build and run the dockerfile, somehow it can't find the mounted directory

[~/workspace/docker-test]$ docker build -t docker-test . && docker run -it -v $(pwd)/source:/usr/source docker-test
[ ] Building 1.6s (5/5) FINISHED
 => [internal] load build definition from Dockerfile                                    0.0s
 => => transferring dockerfile: 36B                                                     0.0s
 => [internal] load .dockerignore                                                       0.0s
 => => transferring context: 2B                                                         0.0s
 => [internal] load metadata for docker.io/library/ubuntu:jammy                         1.1s
 => CACHED [1/2] FROM docker.io/library/ubuntu:jammy@sha256:27cb6e6ccef575a4698b66f5de  0.0s
 => ERROR [2/2] RUN ls /usr/source                                                      0.3s
------
 > [2/2] RUN ls /usr/source:
#5 0.318 ls: cannot access '/usr/source': No such file or directory
------
executor failed running [/bin/sh -c ls /usr/source]: exit code: 2

and yet when I run a shell from within the docker container, I'm able to find it just fine. New Dockerfile:

FROM ubuntu:jammy as prereqs
# RUN ls /usr/source
CMD ["/bin/bash"]

Launches shell from which the mounted volume is perfectly visible.

[~/workspace/docker-test]$ docker build -t docker-test . && docker run -it -v $(pwd)/source:/usr/source docker-test
[ ] Building 0.6s (5/5) FINISHED
 => [internal] load build definition from Dockerfile                                    0.0s
 => => transferring dockerfile: 111B                                                    0.0s
 => [internal] load .dockerignore                                                       0.0s
 => => transferring context: 2B                                                         0.0s
 => [internal] load metadata for docker.io/library/ubuntu:jammy                         0.6s
 => CACHED [1/1] FROM docker.io/library/ubuntu:jammy@sha256:27cb6e6ccef575a4698b66f5de  0.0s
 => exporting to image                                                                  0.0s
 => => exporting layers                                                                 0.0s
 => => writing image sha256:aa762c0645f70aea2a82508f0654abb2aadb27d2abf7e971b5eed85a7e  0.0s
 => => naming to docker.io/library/docker-test                                          0.0s

root@d8b3952e2e80:/# ls /usr/source/
file.txt

I don't understand why it's not visible in the Dockerfile but perfectly visible in the container shell!

CodePudding user response：

RUN ls /usr/source is running on docker build, but CMD is executed on docker run. Volume can't be mounted when image building, so you have to use COPY command inside dockerfile to be able access external resources inside image.

CodePudding user response：

Volumes are only mounted at run-time. As you can see on your build and run commands, the volume specification is only on the docker run command.