[Docs] Fix Docker Walkthrough gateway container name and add supplier staking note

[jblewnormal]

Summary

• Changed incorrect gateway-example Docker container name to gateway under the Configure and run your Gateway Server section of Docker Compose Walkthrough.
• Added a note clarifying one must wait till the full-node has caught up with the current block number before staking their supplier.

I have not run make docusaurus_start as changes are fairly simple and I'm assuming they run on PR submission. Will do so if requested :)

Type of change

Select one or more from the following:

• New feature, functionality or library
• Consensus breaking; add the consensus-breaking label if so. See [Infra] Automatically add the consensus-breaking label #791 for details
• Bug fix
• Code health or cleanup
• Documentation
• Other (specify)

Testing

• Documentation: make docusaurus_start; only needed if you make doc changes
• Unit Tests: make go_develop_and_test
• LocalNet E2E Tests: make test_e2e
• DevNet E2E Tests: Add the devnet-test-e2e label to the PR.

Sanity Checklist

• I have tested my changes using the available tooling
• I have commented my code
• I have performed a self-review of my own code; both comments & source code
• I create and reference any new tickets, if applicable
• I have left TODOs throughout the codebase, if applicable