What is not valid syntax for specifying a Docker volume configuration in Docker Compose?

In Docker Compose, when configuring volumes, the syntax must adhere to specific conventions to ensure proper functionality. Each valid volume definition typically follows the pattern of a host path, a container path, and optionally some configuration flags (like read-only).

The correct option that identifies invalid syntax is the one that includes an extra colon and a trailing dot, which does not conform to volume syntax specifications. In the case of specifying a read-only volume, typically the syntax would include only two or three components: the host path, the container path, and an optional read-only flag. The inclusion of a final colon followed by a dot leads to confusion about volume specification and is not recognized by Docker as a valid configuration.

The other options adhere to valid syntax. For example, defining a volume with just the host path and container path creates a writable volume, while the inclusion of :ro correctly marks the volume as read-only. The last option simply specifies a container path without a host path, which is also valid in some contexts but lacks additional configuration.

Thus, the identification of the specific option as the invalid syntax directly correlates to its failure to follow the expected format used within Docker Compose for volume definitions.