Fix mismatch in 'Build the backend' instructions

[josmperez]

Comment from JackW:

There is a mismatch between amd and arm64 and the docker container which we should likely put a note about here. Previously we forced amd in the docker platform but that causes the backend delve dev env to fail to run on apple silicon.

Also not really sure if this should be added here but we have more than just build:[arch] as available targets. e.g.

| build:backend | build a production build for the current platform | mage -v build:backend |

Maybe we circle back to the whole backend build commands and expand them later on.

[josmperez]

@jackw - When you get a chance, could you review the linked comment above (from you) and let me know if you still think we need to say more about the mismatch between ar64 and docker container, and if so, draft a note on the topic?