This doesn't replace all usage of `-v`, but it's a start.
People sometimes troubleshoot by deleting files (especially bridge
config files). Restarting Synapse with a missing registration.yaml file
for a given bridge, causes the `-v
/something/registration.yaml:/something/registration.yaml:ro` option
to force-create `/something/registration.yaml` as a directory.
When a path that's provided to the `-v` option is missing, Docker
auto-creates that path as a directory.
This causes more breakage and confusion later on.
We'd rather fail, instead of magically creating directories.
Using `--mount`, instead of `-v` is the solution to this.
From Docker's documentation:
> When you use --mount with type=bind, the host-path must refer to an existing path on the host.
> The path will not be created for you and the service will fail with an error if the path does not exist.
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