Answer by isca for How to include files outside of Docker's build context?

This behavior is given by the context directory that the docker or podman uses to present the files to the build process.
A nice trick here is by changing the context dir during the building instruction to the full path of the directory, that you want to expose to the daemon.e.g:

docker build -t imageName:tag -f /path/to/the/Dockerfile /mysrc/path

using /mysrc/path instead of .(current directory), you'll be using that directory as a context, so any files under it can be seen by the build process.
This example you'll be exposing the entire /mysrc/path tree to the docker daemon.
When using this with docker the user ID who triggered the build must have recursively read permissions to any single directory or file from the context dir.

This can be useful in cases where you have the /home/user/myCoolProject/Dockerfile but want to bring to this container build context, files that aren't in the same directory.

Here is an example of building using context dir, but this time using podman instead of docker.

Lets take as example, having inside your Dockerfile a COPY or ADDinstruction which is copying files from a directory outside of your project, like:

FROM myImage:tag......COPY /opt/externalFile ./ADD /home/user/AnotherProject/anotherExternalFile ./...

In order to build this, with a container file located in the /home/user/myCoolProject/Dockerfile, just do something like:

cd /home/user/myCoolProjectpodman build -t imageName:tag -f Dockefile /

Some known use cases to change the context dir, is when using a container as a toolchain for building your souce code.
e.g:

podman build --platform linux/s390x -t myimage:mytag -f ./Dockerfile /tmp/mysrc

or it can be a path relative, like:

podman build --platform linux/s390x -t myimage:mytag -f ./Dockerfile ../../

Another example using this time a global path:

FROM myImage:tag......COPY externalFile ./ADD  AnotherProject ./...

Notice that now the full global path for the COPY and ADD is omitted in the Dockerfile command layers.
In this case the contex dir must be relative to where the files are, if both externalFile and AnotherProject are in /opt directory then the context dir for building it must be:

podman build -t imageName:tag -f ./Dockerfile /opt

Note when using COPY or ADD with context dir in docker:
The docker daemon will try to "stream" all the files visible on the context dir tree to the daemon, which can slowdown the build. And requires the user to have recursively permission from the context dir.This behavior can be more costly specially when using the build through the API. However,with podman the build happens instantaneously, without needing recursively permissions, that's because podman does not enumerate the entire context dir, and doesn't use a client/server architecture as well.
The build for such cases can be way more interesting to use podman instead of docker, when you face such issues using a different context dir.

Some references:

• https://docs.docker.com/engine/reference/commandline/build/
• https://docs.podman.io/en/latest/markdown/podman-build.1.html