Dockerfile changes for Multus Golden Containers in 3.0

All boxes should be checked before merging the PR (just tick any boxes which don't apply to this PR)

[x] The toolchain has been rebuilt successfully (or no changes were made to it)
[x] The toolchain/worker package manifests are up-to-date
[x] Any updated packages successfully build (or no packages were changed)
[x] Packages depending on static components modified in this PR (Golang, *-static subpackages, etc.) have had their Release tag incremented.
[x] Package tests (%check section) have been verified with RUN_CHECK=y for existing SPEC files, or added to new SPEC files
[x] All package sources are available
[x] cgmanifest files are up-to-date and sorted (./cgmanifest.json, ./toolkit/scripts/toolchain/cgmanifest.json, .github/workflows/cgmanifest.json)
[x] LICENSE-MAP files are up-to-date (./SPECS/LICENSES-AND-NOTICES/data/licenses.json, ./SPECS/LICENSES-AND-NOTICES/LICENSES-MAP.md, ./SPECS/LICENSES-AND-NOTICES/LICENSE-EXCEPTIONS.PHOTON)
[x] All source files have up-to-date hashes in the *.signatures.json files
[x] sudo make go-tidy-all and sudo make go-test-coverage pass
[x] Documentation has been updated to match any changes to the build system
[x] If you are adding/removing a .spec file that has multiple-versions supported, please add @microsoft/cbl-mariner-multi-package-reviewers team as reviewer (Eg. golang has 2 versions 1.18, 1.21+)
[x] Ready to merge

What does the PR accomplish, why was it needed?

Renaming the entrypoint to thin_entrypoint from multus-entrypoint to unblock golden container runs.

microsoft / azurelinux