Adding back deleted docker-compose.yml (#1825)

# Description

Please include:
I'm not sure why this `docker-compose.yml` was deleted in the first
place. Nearly half of the instructions in the readme for this repo rely
on this file being there.

Essentially, it's just another way to start up the container using
`docker compose` . This makes the container much more usable for power
users, as they'd be able to configure volumes much easier. Also, instead
of having to remember the whole complicated `docker container run --rm
-d -p 8501:8501 -e STREAMLIT_SERVER_HEADLESS=true -e
"WEBUI_SCRIPT=webui_streamlit.py" -e "VALIDATE_MODELS=false" -v
"${PWD}/outputs:/sd/outputs" --gpus all tukirito/sygil-webui:latest`
command, it's as simple as running `docker compose start` or `up`, since
all of the configs are in the compose file, you wouldn't need anything
else. The compose file has been tested and runs without any issues. The
readme instructions can stay mostly as is.

Additionally, as mentioned before, the readme instructions rely heavily
on docker compose. If the repo creators wish to not include docker
compose, then it should all be omitted from the readme, as it makes this
extremely confusing for any user.

Closes: # (issue)

# Checklist:

- [ ] I have changed the base branch to `dev`
- [ ] I have performed a self-review of my own code
- [ ] I have commented my code in hard-to-understand areas
- [ ] I have made corresponding changes to the documentation

docker-compose.yml

@@ -0,0 +1,29 @@
+version: '3.9'
+
+services:
+  stable-diffusion:
+    container_name: sd-webui
+    build:
+      context: .
+      dockerfile: Dockerfile
+    env_file: .env_docker
+    environment:
+      PIP_EXISTS_ACTION: w
+    volumes:
+      - .:/sd
+      - ./outputs:/sd/outputs
+      - ./model_cache:/sd/model_cache
+      - conda_env:/opt/conda
+      - root_profile:/root
+    ports:
+      - '7860:7860'
+      - '8501:8501'
+    deploy:
+      resources:
+        reservations:
+          devices:
+            - capabilities: [ gpu ]
+
+volumes:
+  conda_env:
+  root_profile: