Fix & clean up Dockerfile (#4502)
Fix & clean up Dockerfile (#4502)

We no longer need `_InternalSwiftSyntaxParser`, but we do need
`libswiftCore.so`.

Fixes https://github.com/realm/SwiftLint/issues/4501

Add a more useful validation test at the end of the Dockerfile.
In addition to running `swiftlint version`, also actually run the linter
which will exercise the SourceKit codepaths.
realm/SwiftLint,realm/SwiftLint,realm/SwiftLint,realm/SwiftLint