Docker container - terraform modules symlink should be created if it doesn't exist

For the setup-get-symlinks.sh script used to link environment configurations in the Dockerfile, the current -d test for the terraform modules symlink results in a situation where the symlink is never created - and will result in a confusing "Unreadable module directory" on first runs:

│ Error: Unreadable module directory
│
│ Unable to evaluate directory symlink: lstat ../../modules: no such file or directory
╵

The simplest correction for this is likely change the test to ! -e and just create the symlink if an existing modules directory/symlink/file doesn't already exist