📝 Add Kubernetes warning, when to use this image (#245)

README.md

@@ -50,6 +50,74 @@ It is very similar to **tiangolo/uwsgi-nginx-flask**, so you can still use many
 
 **Docker Hub image**: [https://hub.docker.com/r/tiangolo/uwsgi-nginx-flask/](https://hub.docker.com/r/tiangolo/uwsgi-nginx-flask/)
 
+## 🚨 WARNING: You Probably Don't Need this Docker Image
+
+You are probably using **Kubernetes** or similar tools. In that case, you probably **don't need this image** (or any other **similar base image**). You are probably better off **building a Docker image from scratch**.
+
+---
+
+If you have a cluster of machines with **Kubernetes**, Docker Swarm Mode, Nomad, or other similar complex system to manage distributed containers on multiple machines, then you will probably want to **handle replication** at the **cluster level** instead of using a **process manager** in each container that starts multiple **worker processes**, which is what this Docker image does.
+
+In those cases (e.g. using Kubernetes) you would probably want to build a **Docker image from scratch**, installing your dependencies, and running **a single process** instead of this image.
+
+For example, using [Gunicorn](https://gunicorn.org/) you could have a file `app/gunicorn_conf.py` with:
+
+```Python
+# Gunicorn config variables
+loglevel = "info"
+errorlog = "-"  # stderr
+accesslog = "-"  # stdout
+worker_tmp_dir = "/dev/shm"
+graceful_timeout = 120
+timeout = 120
+keepalive = 5
+threads = 3
+```
+
+And then you could have a `Dockerfile` with:
+
+```Dockerfile
+FROM python:3.9
+
+WORKDIR /code
+
+COPY ./requirements.txt /code/requirements.txt
+
+RUN pip install --no-cache-dir --upgrade -r /code/requirements.txt
+
+COPY ./app /code/app
+
+CMD ["gunicorn", "--conf", "app/gunicorn_conf.py", "--bind", "0.0.0.0:80", "app.main:app"]
+```
+
+You can read more about these ideas in the [FastAPI documentation about: FastAPI in Containers - Docker](https://fastapi.tiangolo.com/deployment/docker/#replication-number-of-processes) as the same ideas would apply to other web applications in containers.
+
+## When to Use this Docker Image
+
+### A Simple App
+
+You could want a process manager running multiple worker processes in the container if your application is **simple enough** that you don't need (at least not yet) to fine-tune the number of processes too much, and you can just use an automated default, and you are running it on a **single server**, not a cluster.
+
+### Docker Compose
+
+You could be deploying to a **single server** (not a cluster) with **Docker Compose**, so you wouldn't have an easy way to manage replication of containers (with Docker Compose) while preserving the shared network and **load balancing**.
+
+Then you could want to have **a single container** with a **process manager** starting **several worker processes** inside, as this Docker image does.
+
+### Prometheus and Other Reasons
+
+You could also have **other reasons** that would make it easier to have a **single container** with **multiple processes** instead of having **multiple containers** with **a single process** in each of them.
+
+For example (depending on your setup) you could have some tool like a Prometheus exporter in the same container that should have access to **each of the requests** that come.
+
+In this case, if you had **multiple containers**, by default, when Prometheus came to **read the metrics**, it would get the ones for **a single container each time** (for the container that handled that particular request), instead of getting the **accumulated metrics** for all the replicated containers.
+
+Then, in that case, it could be simpler to have **one container** with **multiple processes**, and a local tool (e.g. a Prometheus exporter) on the same container collecting Prometheus metrics for all the internal processes and exposing those metrics on that single container.
+
+---
+
+Read more about it all in the [FastAPI documentation about: FastAPI in Containers - Docker](https://fastapi.tiangolo.com/deployment/docker/), as the same concepts apply to other web applications in containers.
+
 ## Examples (simple project templates)
 
 * **`python3.8`** tag: general Flask web application:
@@ -66,11 +134,19 @@ It is very similar to **tiangolo/uwsgi-nginx-flask**, so you can still use many
 
 ## General Instructions
 
-You don't have to clone this repo, you should be able to use this image as a base image for your project with something in your `Dockerfile` like:
+You don't have to clone this repo.
+
+You can use this image as a base image for other images.
+
+Assuming you have a file `requirements.txt`, you could have a `Dockerfile` like this:
 
 ```Dockerfile
 FROM tiangolo/uwsgi-nginx-flask:python3.9
 
+COPY ./requirements.txt /app/requirements.txt
+
+RUN pip install --no-cache-dir --upgrade -r /app/requirements.txt
+
 COPY ./app /app
 ```