Use tagged release of parsec-tool when available

[paulhowardarm]

Summary
The Dockerfile is currently checking out and building parsec-tool at a specific commit hash. This is because it needs to use the encrypt command, which was only added recently. At time of writing, this command is not available in any released version of the tool. Once a released version becomes available, we should checkout that tagged version rather than a commit hash. There is a TODO comment in the Dockerfile that can be removed once we have done this.