Scaffold incremental Terraform foundations for docker and proxmox

infrastructure/terraform/modules/docker_container_placeholder/README.md

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+# docker_container_placeholder module
+
+Placeholder module directory for future shared Docker container patterns.
+
+This is intentionally empty during initial import-first adoption.
+Keep per-container resources explicit in `../../docker/main.tf` until stable patterns emerge.

infrastructure/terraform/modules/proxmox_vm_placeholder/README.md

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+# proxmox_vm_placeholder module
+
+Placeholder module directory for future shared Proxmox VM patterns.
+
+This is intentionally empty until provider schemas, import ID formats,
+and non-destructive reconciliation strategy are validated.