how does docker-compose.yml and dockerfile works together - docker

Question

From my understanding,
Dockerfile is like the config/recipe for creating the image, while docker-compose is used to easily create multiple containers which may have relationship, and avoid creating the containers by docker command repeatedly.
There are two files.
Dockerfile
FROM node:lts-alpine
WORKDIR /server
COPY package*.json ./
RUN npm install
COPY . .
EXPOSE 3030
CMD ["npm", "run", "dev"]
docker-compose.yml
version: '2.1'
services:
test-db:
image: mysql:5.7
environment:
- MYSQL_ALLOW_EMPTY_PASSWORD=true
- MYSQL_USER=admin
- MYSQL_PASSWORD=12345
- MYSQL_DATABASE=test-db
volumes:
- ./db-data:/var/lib/mysql
ports:
- 3306:3306
test-web:
environment:
- NODE_ENV=local
#- DEBUG=*
- PORT=3030
image: node:lts-alpine
build: ./
command: >
npm run dev
volumes:
- ./:/server
ports:
- "3030:3030"
depends_on:
- test-db
Question 1
When I run docker-compose up --build
a. The image will be built based on Dockerfile
b. What's then?
Question 2
test-db:
image: mysql:5.7
test-web:
environment:
- NODE_ENV=local
#- DEBUG=*
- PORT=3030
image: node:lts-alpine
I am downloading the image for dockerhub with above code, but why and when do I need the image created in --build?
Question 3
volumes:
- ./db-data:/var/lib/mysql
Is this line means that the data is supposed to store at memory in location /var/lib/mysql, while I copy this data in my working directory ./db-data?
Update
Question 4
build: ./
What is this line for?

Answer 1

It is recommended to go through the Getting Started, most of your questions would be solved.
Let me try to highlight some of those to you.
The difference between Dockerfile and Compose file
Docker can build images automatically by reading the instructions from a Dockerfile
Compose is a tool for defining and running multi-container Docker applications
The main difference is Dockerfile is used to build an image while Compose is to build and run an application.
You have to build an image by Dockerfile then run it by Compose
After you run docker-compose up --build the image is built and cached in your system, then Compose would start the containers defined by docker-compose.yml
If you specify the image then it would be download while built if specify the build: ./
Volumes are the preferred mechanism for persisting data generated by and used by Docker containers., Images are read-only, and all editing for a container would be destroyed after it's deleted, so you have to use Volumes if you want to persistent data.
Remember, Doc is always your friend.