Updating install script and Dockerfile to use version ARG

[erikaheidi]

This updates the server install to check for a default value of "latest", and updates the Dockerfile to use an ARG to set the server version. You can now specify the server version at build time:

docker build --build-arg VERSION=1.21.4 . -t guardcraft-1.21.4

[Copilot]

Pull Request Overview

Updates the installation instructions and Docker build process to support specifying the server version at build time.

• Updated usage instructions in the README to mention the build argument for setting the server version.
• Provided a Docker build command example showing the use of the build argument.

Files not reviewed (2)

• Dockerfile: Language not supported
• server-install.sh: Language not supported

Comments suppressed due to low confidence (1)

README.md:45

• The version mentioned in the client connection instructions (1.21.5 Pre-Release 3) does not match the version shown in the build command (1.21.4). Consider using a consistent version across the documentation.

Please note the client must be running the same version of the server software (in this case, `1.21.5 Pre-Release 3`)