Break down `rodan-python3-celery:nightly` image in `backend-django` (previously `rodan-main`)

Let's break down this line into a builder image

https://github.com/DDMAL/Rodan-lite/blob/4fb5313bc667b5f0e1703b849cb3f7224f72ed1d/backend/rodan-main/Dockerfile#L5

---

I'm not sure why this is the case. I'm assuming that Rodan resuses the same base everywhere but it's very confusing to know which one is an actual service and which one is just a base image.

I've found the reason! In `settings.py`, we have this part below. This explains why Rodan's `rodan-main` and `celery` (and probably `python3-celery` and `gpu-celery` in the past) all use the exact same base image. The environment variable `CELERY_JOB_QUEUE` is specified in `docker-compose.yml` file

```py
# Job Packages
RODAN_JOB_QUEUE = os.getenv("CELERY_JOB_QUEUE")
RODAN_JOB_PACKAGES = []

BASE_JOB_PACKAGES = [base_jobs for base_jobs in allJobs['BASE_JOB_PACKAGES']]
RODAN_PYTHON3_JOBS = [py3_jobs for py3_jobs in allJobs['RODAN_PYTHON3_JOBS']]
RODAN_GPU_JOBS = [gpu_jobs for gpu_jobs in allJobs['RODAN_GPU_JOBS']]

if RODAN_JOB_QUEUE == "None" or RODAN_JOB_QUEUE == "celery":
# All the jobs must be registered in the database, so all jobs must be here.
RODAN_JOB_PACKAGES += BASE_JOB_PACKAGES
RODAN_JOB_PACKAGES += RODAN_PYTHON3_JOBS
RODAN_JOB_PACKAGES += RODAN_GPU_JOBS
elif RODAN_JOB_QUEUE == "Python3":
RODAN_JOB_PACKAGES += BASE_JOB_PACKAGES
RODAN_JOB_PACKAGES += RODAN_PYTHON3_JOBS
elif RODAN_JOB_QUEUE == "GPU":
RODAN_JOB_PACKAGES += BASE_JOB_PACKAGES
RODAN_JOB_PACKAGES += RODAN_GPU_JOBS
elif RODAN_JOB_QUEUE == "Docs":
RODAN_JOB_PACKAGES = []
else:
raise EnvironmentError(
"An environment was not built for that specific rodan job-queue yet.\n"
"\tBuild one and try again.\n"
"\t{}\n".format(RODAN_JOB_QUEUE)
)
```
_Originally posted by @notkaramel in [#11](https://github.com/DDMAL/Rodan-lite/issues/11#issuecomment-3055326230)_