docs: fix Docker image cleanup command

[nvcc1701]

Summary

• replace grep/awk parsing of docker images -a output with Docker's --filter and --format "{{.ID}}" options
• update both local and remote Docker docs that used the same cleanup command

Testing

• rg -n "docker images -a \| grep|awk '\{print \$3\}'" content/docs returns no matches
• bun run build passes

Note: build emitted existing remote image-size warnings for GitHub-hosted images, then completed successfully.

[vercel]

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