Improve plugin loading, and include sample plugin with docker container

[mattjohnsonpint]

• Updates the dockerfile to include the runtime and the example hmplugin1 plugin, and load it by default.
• Adds a new parameter --plugins which can be used to set the path to the plugins directory. Used by the dockerfile.
• Loads all plugins in the plugins directory, using either <pluginsdir>/myplugin/build/debug.wasm, or <pluginsdir>/myplugin.wasm, allowing both for local development and production usage.
• Updates the readme files for docker instructions.