Bottom
同 supplies the sound — tóng drifted to tǒng with a tone shift. Same phonetic in
桶 tǒng (bucket),
铜 tóng (copper),
通 tōng (through),
痛 tòng (pain).
同 also depicts an enclosure (
口) wrapping content, faintly reinforcing the hollow-cylinder picture, but the role here is primarily phonetic.