It seems that 1.3.0's COPY command doesn't copy relative symlinks anymore. As far as I know 1.2.0 did.

My directory layout looks as follows:

assets/
   a/
      x/
      y/
      z/
   b/
   c/
      y/ --> symlink to ../a/y
   d/
Dockerfile

E.g. the symlinks are not outside of the root directory that contains the Dockerfile.

As far as I can remember (I haven't verified this), on 1.2.0 the c/y symlink to ../a/y was kept in tact. In 1.3.0 the symlink is simply gone.