Apologies for the breaking change. This was unexpected.
We’ve just rolled out an update to the 14.04 image so builds that ran successfully with 5.6 should now run without changes on 5.7 (unless you are using MySQL features subject to breaking changes from MySQL 5.6 to 5.7).
So our recommendation is to run with the preinstalled 5.7 and only revert to 5.6 if your database is incompatible with 5.7.