[BUG] - container securityContext and read-only filesystem

[DrPsychick]

Resolve issues from Trivy scan

Affects all deployments: https://github.com/Mailu/helm-charts/security/code-scanning

Root file system is not read-only

Artifact: charts/mailu/templates/postfix/deployment.yaml Type: helm Vulnerability KSV014 Severity: HIGH Message: Container 'postfix' of Deployment 'mailu-postfix' should set 'securityContext.readOnlyRootFilesystem' to true Link: KSV014

Default security context configured

Artifact: charts/mailu/templates/postfix/deployment.yaml Type: helm Vulnerability KSV118 Severity: HIGH Message: container mailu-postfix in namespace is using the default security context Link: KSV118