docker: Update Dockerfiles

[yolile]

I tried to build the frontend Dockerfile locally but I got the error described here: nodejs/docker-node#384

The solution is what is described now in the base image documentation: https://github.com/nodejs/docker-node/blob/main/docs/BestPractices.md#node-gyp-alpine

Then, also in the frontend Dockerfile, the node:14-alpine base image should be used, as the dependency node-saas is used in its version 4.14 which is compatible only with node 14

We should also update the frontend and backend Dockerfiles to be run as a non-root user as open-contracting/deploy#320