Every two months, SQL Server images in the virtual machine gallery are updated with the latest Windows and Linux updates. For Windows images, this includes any updates that are marked important in Windows Update, including important SQL Server security updates and service packs. For Linux images, this includes the latest system updates. SQL Server cumulative updates are handled differently for Linux and Windows. For Linux, SQL Server cumulative updates are also included in the refresh. But at this time, Windows VMs are not updated with SQL Server or Windows Server cumulative updates.
Source : Azure Documentation FAQ Link