fix: remove spurious '- ' prefix from additional_post_build_commands

The packer-pipeline internal buildspec template already wraps the value
in '- {{ additional_post_build_commands }}', so prefixing the value with
'- ' caused YAML_FILE_ERROR (nested list) in CodeBuild build #8.

csvd_config_packer.hcl

@@ -43,7 +43,7 @@ packer_pipeline {
   buildspec_template   = "buildspec.yml.j2"  // Buildspec template file
 
   // Post-build commands to push Docker image to ECR
-  additional_post_build_commands  = "- docker push ${AWS_ACCOUNT_ID}.dkr.ecr.${AWS_REGION}.amazonaws.com/${ECR_REPOSITORY}:${IMAGE_TAG}"
+  additional_post_build_commands  = "docker push ${AWS_ACCOUNT_ID}.dkr.ecr.${AWS_REGION}.amazonaws.com/${ECR_REPOSITORY}:${IMAGE_TAG}"
 
   // Exclude patterns for zip creation
   exclude_dirs     = [