Update Nethermind Arbitrum image to the latest 0.2.1 release

[wurdum]

No description provided.

[Copilot]

Pull request overview

Updates the default Nethermind Arbitrum Docker image tag used by the local Docker Compose setup to the latest 0.2.1 release, keeping the example environment file in sync.

Changes:

• Bump nethermind/nethermind-arbitrum default image tag from 0.2.0 to 0.2.1 in docker-compose.yml.
• Update .env.example to match the new default NETHERMIND_IMAGE tag (0.2.1).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
docker-compose.yml	Updates the default Nethermind Arbitrum container image tag to 0.2.1.
.env.example	Aligns the example NETHERMIND_IMAGE value with the updated Compose default (0.2.1).

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.97%. Comparing base (abbd401) to head (9716267).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #961   +/-   ##
=======================================
  Coverage   74.97%   74.97%           
=======================================
  Files         244      244           
  Lines       15111    15111           
  Branches     2293     2293           
=======================================
  Hits        11330    11330           
  Misses       2924     2924           
  Partials      857      857           

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