🐛(docker) build Docker images with an unprivileged user

This is a major issue. Docker Images were built and published with a
root user in the CI.

if a user manages to break out of the application running as root in the
container, he may gain root user access on host. In addition, configuring
container to user unprivileged is the best way yo prevent privilege
escalation attacks.

We mitigated this issue by creating a new environment variable DOCKER_USER.
DOCKER_USER is set with id -u and id -g outputs. Then, it is passed as a
build-args when running docker/build-push-action steps.

.github/workflows/docker-hub.yml

@@ -11,6 +11,9 @@ on:
     branches:
       - 'main'
 
+env:
+  DOCKER_USER: 1001:127
+
 jobs:
   build-and-push-backend:
     runs-on: ubuntu-latest
@@ -40,6 +43,7 @@ jobs:
         with:
           context: .
           target: production
+          build-args: DOCKER_USER=${{ env.DOCKER_USER }}:-1000
           push: ${{ github.event_name != 'pull_request' }}
           tags: ${{ steps.meta.outputs.tags }}
           labels: ${{ steps.meta.outputs.labels }}
@@ -72,6 +76,7 @@ jobs:
         with:
           context: .
           target: frontend
+          build-args: DOCKER_USER=${{ env.DOCKER_USER }}:-1000
           push: ${{ github.event_name != 'pull_request' }}
           tags: ${{ steps.meta.outputs.tags }}
           labels: ${{ steps.meta.outputs.labels }}