Copy config under a volume for Docker (similar to a ConfigMap for K8S). (#914)

This emulates the K8S ConfigMap behavior on Docker by using a regular volume.

If a directory exists under `config/` which matches a named volume, the contents will be copied to the volume on `create` (provided the destination volume is empty).  That is, rather than a ConfigMap, it is essentially a "config volume".

For example, with a compose file like:

```
version: '3.7'
services:
  caddy:
    image: cerc/caddy-ethcache:local
    restart: always
    volumes:
      - caddyconfig:/etc/caddy:ro
volumes:
  caddyconfig:
```

And a directory:

```
❯ ls stack-orchestrator/config/caddyconfig/
Caddyfile
```

After `laconic-so deploy create --spec-file caddy.yml --deployment-dir /srv/caddy` there will be:

```
❯ ls /srv/caddy/data/caddyconfig
Caddyfile
```

Mounted at `/etc/caddy`

Reviewed-on: #914
Reviewed-by: David Boreham <dboreham@noreply.git.vdb.to>
Co-authored-by: Thomas E Lackey <telackey@bozemanpass.com>
Co-committed-by: Thomas E Lackey <telackey@bozemanpass.com>

stack_orchestrator/deploy/deployment_create.py

@@ -520,6 +520,16 @@ def create_operation(deployment_command_context, spec_file, deployment_dir, netw
                 if os.path.exists(source_config_dir):
                     destination_config_dir = deployment_dir_path.joinpath("configmaps", configmap)
                     copytree(source_config_dir, destination_config_dir, dirs_exist_ok=True)
+        else:
+            # TODO: We should probably only do this if the volume is marked :ro.
+            for volume_name, volume_path in parsed_spec.get_volumes().items():
+                source_config_dir = resolve_config_dir(stack_name, volume_name)
+                # Only copy if the source exists and is _not_ empty.
+                if os.path.exists(source_config_dir) and os.listdir(source_config_dir):
+                    destination_config_dir = deployment_dir_path.joinpath(volume_path)
+                    # Only copy if the destination exists and _is_ empty.
+                    if os.path.exists(destination_config_dir) and not os.listdir(destination_config_dir):
+                        copytree(source_config_dir, destination_config_dir, dirs_exist_ok=True)
 
     # Delegate to the stack's Python code
     # The deploy create command doesn't require a --stack argument so we need to insert the