Force UTF-8 encoding when reading Groovy files

Previously, when opening Groovy files to inspect their contents, we
relied on the default encoding of the runtime.  This caused issues
when reading a file that had a different encoding than the default.
This change forces the encoding to be UTF-8 at all times.

[#54176012][#5]
Merge 54176012-groovy-container-error to master

[#54176012]