Add DNS utilities and Exchange Online administration runtime

[Josh-XT]

Summary

• Bundle DNS utilities (dig/host/nslookup) so email and DNS diagnostics do not stall on a missing binary.
• Install checksum-verified PowerShell 7.4.20 and pinned ExchangeOnlineManagement 3.10.1 for the companion WorkConductor Microsoft 365 Admin extension.
• Keep installation in a late image layer; support amd64/arm64 downloads and smoke-test import/AccessToken availability as the unprivileged sandbox user.

Validation

• Full docker build succeeded: joshxt/safeexecute:mail-admin-test.
• Unprivileged dig version and actual Exchange module import passed during the build.
• Seven WorkConductor PowerShell contract tests passed in this image with networking disabled (read/create/update, failed readback, unknown operation and secret redaction).
• bash -n scripts/install-mail-tools.sh passed.

Deployment

Publish/pull this image before using Exchange abilities in WorkConductor. Recycle existing sandboxes only after their active tasks finish; none were restarted during this work. No account credentials, DNS changes or tenant mutations were used in testing. Linux amd64 was built locally; arm64 was not executed here.

Companion WorkConductor PR: https://github.com/Josh-XT/WorkConductor/pull/366