Allow following symlinks when building docker images

[sherifnada]

What

This PR enables including symlinks in Docker's build context when building a connector image. This is helpful in cases where we want to use symlinks to depend on local versions of other packages in the repo.

In the consuming build.gradle, we only need to add a block

airbyteDocker {
  followSymlinks = true
}

to use this change. All current consumers should be unaffected since by default symlinks are not followed.

This is supporting work for @gordalina 's Elixir GA source

Reading order

1. build_image.sh
2. airbyte-docker.gradle