add Jenkinsfile for build ci pipeline

[fpichler]

Add Jenkinsfile for build CI pipeline and align environment profiles

Description

This PR introduces a declarative Jenkinsfile at the root of the repository to build and push the celements-admin-frontend Docker image to the GitHub Container Registry (ghcr.io). It also introduces the celdev environment profile and aligns/updates the integration environment configuration.

Proposed Changes

Jenkins Pipeline

• Added Jenkinsfile:
  • Configures a multi-stage declarative pipeline (Checkout -> Build Frontend Docker Image -> Push to GHCR).
  • Generates and tags the image using the current aligned version (7.1) as well as the short Git SHA.
  • Assumes credentials with ID ghcr-credentials are configured in Jenkins for authorization with ghcr.io.

Environment Configurations (celements-admin-frontend)

• Added .env.celdev:
  • Configured environment settings for the celdev profile pointing to https://bellis.celdev.sneakapeek.ch and Keycloak realm celdev.
• Renamed & Updated .env.int ➡️ .env.celint:
  • Aligned the filename with the celint naming standard.
  • Corrected VITE_CEL_API_URL to point to the demospace.ch domain: https://bellis.celint.demospace.ch (previously pointed to sneakapeek.ch).

[msladek]

I think we should make use of the Jenkins Docker Pipeline integration instead of raw docker build/docker login shell commands:

https://www.jenkins.io/doc/book/pipeline/docker/