Dockerfile ADD variable not expandable - docker

Question

I am setting up a docker image, in the dockerfile I have an ADD command where source of the ADD command is a variable.
Dockerfile takes a build argument, I want to use that arg as source of the ADD command.
But ADD command is not expanding the variable and I get an error
Please share any workaround that comes in your mind
FROM ubuntu
ARG source_dir
RUN echo ${source_dir}
ADD ${source_dir} ./ContainerDir
Build command
docker build . -t image --build-arg source_dir=/home/john/Desktop/
data
Error
Step 3/3 : ADD ${source_dir} ./ContainerDir ADD failed: stat /var/lib/docker/tmp/docker-builder311119108/home/john/Desktop/
data: no such file or directory
However, the directory (/home/john/Desktop/
data) exists

Answer 1

From the error message, the variable expanded and complained that you don't have the path in your build context:
stat /var/lib/docker/tmp/docker-builder311119108/a/b/c: no such file or directory
In your example, the build context is . (the current directory) so you need a/b/c in the current directory for this to not error. That also need to not be in any ./.dockerignore file if you have one.
From your second edit:
docker build . -t image --build-arg source_dir=/home/john/Desktop/data
It looks like you are trying to include a directory inside your build from outside of the build context. That is explicitly not allowed in docker builds. All files needed for the ADD and COPY commands need to be included in your context, and the entire content of the context is sent to the build server in the first step, so you want to keep this small (rather than sending the entire home directory). The source is always relative to this context, so /home is looking for ./home since your context is . in the build command.
The fix is to move the data directory to be a sub directory of . where you are building your docker images. You can also switch to COPY since there is no functionality of ADD that you need.
Disclaimer: there are two pieces of over simplification here:
The COPY command can include files from different contexts using the --from option to COPY.
The entire context is sent before the build starts with the classic build command. The newer BuildKit implementation is much more selective about how much and what parts of the context to send.