[keyserver] Run source-nvm.sh using bash in Docker container

Summary:
Relevant Linear issue with full context here. This diff modifies the Dockerfile in keyserver to execute source-nvm.sh in the environment specified by the shebang in source-nvm.sh instead of in the current environment of the container, which uses sh.

Read more here.

Thank you @atul for your help on this!

Test Plan:
Ran the Docker keyserver build locally and it worked with no errors.

Reviewers: atul, ashoat

Reviewed By: ashoat

Subscribers: palys-swm, Adrian

Differential Revision: https://phab.comm.dev/D4429