remove manual cp from Dockerfiles

[spypsy]

Description

• add build:link as part of building bb.js and circuits.js so no manual cp step is needed in Dockerfiles that depend on them

Checklist:

• I have reviewed my diff in github, line by line.
• Every change is related to the PR description.
• I have linked this pull request to the issue(s) that it resolves.
• There are no unexpected formatting changes, superfluous debug logs, or commented-out code.
• The branch has been merged or rebased against the head of its merge target.
• I'm happy for the PR to be merged at the reviewer's next convenience.

[spalladino]

@spypsy I think we can close this after #229, right?