Fix error when running `make` / `docker-compose up`

This fixes an error when running the mobilizon_api container (either
with `make` or `docker-compose up`): the command `mix phx.server`
was run first in the mobilizon_api container, but it
made the container restart every ~10 seconds because of missing
dependencies errors. As a result it became impossible to install
the depencies using
`docker-compose exec api sh -c "cd js && yarn install && cd ../"`,
because that would give this error:
`Error response from daemon: Container
8ff288ad74565bb0773f9b5f8fb39ab62925c16ea0ca77c5d2579cc3013a8464 is
restarting, wait until the container is running`.

This commit fixes this error by running the dependency installation
commands (and compilation and db migration) before `mix phx.server`.