How to make a symlinked folder appear as a normal folder

I don't have much experience with docker so I can't promise this will work but one choice would be to mount the directory instead of linking to it:

$ cd projects/app1
$ mkdir shared
$ sudo mount -o bind ../shared shared/

That will attach ../shared to ./shared and should be completely transparent to the system. As explained in man mount:

The bind mounts.

Since Linux 2.4.0 it is possible to remount part of the file hierarchy somewhere else. The call is:

mount --bind olddir newdir

or by using this fstab entry:

/olddir /newdir none bind

After this call the same contents are accessible in two places.

This issue has come up repeatedly in the Docker community. It basically violates the requirement that a Dockerfile be repeatable if you run it or I run it. So I wouldn't expect this ability, as described in this ticket: Dockerfile ADD command does not follow symlinks on host #1676.

So you have to conceive of a different approach. If you look at this issue: ADD to support symlinks in the argument #6094, a friend of ours from U&L (@Patrick aka. phemmer) provides a clever workaround.

$ tar -czh . | docker build -

This tells tar to dereference the symbolic links from the current directory, and then pipe them all to the docker build - command.

-c, --create
       create a new archive

-h, --dereference
       follow symlinks; archive and dump the files they point to

-z, --gzip, --gunzip --ungzip