MSSQL use case needs a driverClassName

[izgeri]

At current the application config does NOT include a spring.datasource.driverClassName field, and in testing the Pet Store communicating to an RDS MSSQL DB with Secretless, we've discovered this is required.

AC:

• Update the app config to allow specifying the spring.datasource.driverClassName via an env var
• Update the README to let users know that for MSSQL it requires a specific value
• Update the DockerHub with this information as well

[sgnn7]

Marking this as potentially invalid as it seems like it may be a symptom of incomplete DB setup when done over secretless-broker.