docs: clarify both SA_PASSWORD vars must be updated

Update README to mention that both SA_PASSWORD and MSSQL_SA_PASSWORD
must be updated in docker-compose.yml when changing the password,
matching the fix applied to go-mssqldb PR #317.

Author: dlevy-msft-sql

.devcontainer/README.md

@@ -187,7 +187,7 @@ The `setup.sql` script in `.devcontainer/mssql/` is executed automatically when
 
 To change the SQL Server password:
 
-1. Update `MSSQL_SA_PASSWORD` in `docker-compose.yml`
+1. Update both `SA_PASSWORD` and `MSSQL_SA_PASSWORD` in `docker-compose.yml` (these must match)
 2. Update `SQLCMDPASSWORD` in `devcontainer.json` (both `remoteEnv` and `go.testEnvVars` sections)
 3. Update the password in the `mssql.connections` settings in `devcontainer.json`