Switch to clang?

Not sure if there's a specific reason for using GCC, but I recently dockerized [Elements](https://github.com/ElementsProject/elements), a fork of Bitcoin Core, and switching to clang saved about an hour of build time. So if you're looking for faster builds, maybe using clang would make sense.

Start by inspecting the repository's Docker build configuration and determine where GCC is selected for the Elements image. Compare the current build behavior with the reported clang approach, including the supported image targets, and confirm that the resulting images still build successfully with the intended faster compiler.

Written by the indexing model from the issue text.