fix: add SOAP extension to PHP image

[vitormattos]

Summary

• add the missing SOAP extension to the PHP image used by the Akaunting docker environment
• keep the change isolated to the PHP image build

How to test

• rebuild the akaunting.php image
• run \ inside the container and confirm SOAP is loaded

Notes

• this PR follows the required branch/PR workflow and should only be merged after all CI checks pass