chore: remove Docker local dev feature completely

### What is this about?

Following up on the quick flaky CI fix https://github.com/MetaMask/metamask-mobile/pull/26693 :
If the Docker image is not passing checks and is not used, it's better to completely remove it from the codebase.
This was introduced for local dev but was never really used and only allowed devs without macOS to build. Eventually, we don't really have these devs, everyone is equiped.

### Scenario

_No response_

### Design

_No response_

### Technical Details

- remove Dockerfile
- remove docker workflow
- update doc
- check nothing is impacted

### Threat Modeling Framework

- no big risk aside form perhaps an isolated contributor not being able to use the image anymore. Note that this doesn't prevent them to build an image, it would just not be shipped in the repo anymore.

### Acceptance Criteria

- removed Dockerfile
- removed docker workflow
- updated doc
- nothing is impacted

### Stakeholder review needed before the work gets merged

- [x] Engineering (needed in most cases)
- [ ] Design
- [ ] Product
- [ ] QA (automation tests are required to pass before merging PRs but not all changes are covered by automation tests - please review if QA is needed beyond automation tests)
- [ ] Security
- [ ] Legal
- [ ] Marketing
- [ ] Management (please specify)
- [ ] Other (please specify)

### References

https://github.com/MetaMask/metamask-mobile/pull/26693

Start by locating the Dockerfile, Docker workflow, and documentation references in the repository, then review PR #26693 for context. Remove the specified Docker artifacts, update the affected documentation, and verify that no remaining references or checks are impacted.

Written by the indexing model from the issue text.