ruby: run_with*.sh should use realpath for shared folder (#292)

On macOS, the Docker app uses the canonical path (symlinks resolved) for
shared folders.

This prevents sharing the directory containing the ruby scripts if the
path of the current working directory contains any directory symlinks.

Use realpath instead of pwd in these scripts, falling back to using pwd
when realpath is not available.

ruby/run_with_jekyll.sh

@@ -5,7 +5,7 @@ if ! docker inspect ruby_with_jekyll > /dev/null 2>&1; then
 fi
 
 docker run -it --rm --name jekyll \
-    --volume=$PWD:/srv/jekyll \
+    --volume="$(realpath 2>/dev/null || pwd)":/srv/jekyll \
     -p 1234:1234 \
      ruby_with_jekyll \
      $1

ruby/run_with_liquid.sh

@@ -5,6 +5,6 @@ if ! docker inspect ruby_with_liquid > /dev/null 2>&1; then
 fi
 
 docker run -it --rm --name liquid \
-    --volume=$PWD:/srv/liquid \
+     --volume="$(realpath 2>/dev/null || pwd)":/srv/liquid \
      ruby_with_liquid \
      $1