Is there a specific command/clause in the DBCC SHRINKDATABASE command where I can force a database to release free space to the operating system ?
yes . use TRUNCATEONLY.
Causes any unused space in the data files to be released to the operating system and shrinks the file to the last allocated extent, reducing the file size without moving any data. No attempt is made to relocate rows to unallocated pages. target_percent is ignored when TRUNCATEONLY is used.