docker-compose yaml.parser.ParserError

Question

With the following docker-compose.yml I always get a syntax error I can't explain (I don't see the difference in lines 2 and 3 between the two docker-compose.ymls)

---
version: '2'
  services:
    app-module:
      container_name: app-module:
      env_file: ./app-module:.env
      image: registry.x/app/app-module:latest
      network_mode: "bridge"
        ports:
        - "30303:30303"
      volumes:
      - type: volume
        source: node-volume
        target: /datadir
      - ./data:/data
      - ./log:/log

Error message:

ERROR: yaml.parser.ParserError: while parsing a block mapping
  in "./docker-compose.yml", line 2, column 1
expected <block end>, but found '<block mapping start>'
  in "./docker-compose.yml", line 3, column 3

I don't see any syntax differences to other working files.

That's the working docker-compose.yml I used as the inspiration of my file:

---
version: '2'
services:
  app-node:
    container_name: app-node
    env_file: ./app-node.env
    image: registry.x/group/app-node:latest
    network_mode: "bridge"
    ports:
      - "7990:7990"
      - "7999:7999"
    volumes:
      - ./data:/data
      - ./log:/log

Proof:

$ docker-compose config
services:
  app-node:
    container_name: app-node
    environment: {}
    image: registry.x/group/app-node:latest
    network_mode: bridge
    ports:
    - 7990:7990/tcp
    - 7999:7999/tcp
    volumes:
    - ...app-node/Test/data:/data:rw
    - ...app-node/Test/log:/log:rw
version: '2.0'

Answer 1

Spaces matter in YAML. You have two spaces before services: that are not supposed to be there. You're telling YAML that services is in version but version already has a value.

It's the difference between:

foo: bar
  in_foo: bar

which will not work because in_foo is in foo, and:

foo: bar
not_in_foo: bar

that will work because not_in_bar is not in foo.

Alternatively, this would be valid syntax (but then docker-compose will fail because it expects a string in version):

version:
  services:
    foo: bar