[syntax] YAML: Do I need quotes for strings in YAML?

I am trying to write a YAML dictionary for internationalisation of a Rails project. I am a little confused though, as in some files I see strings in double-quotes and in some without. A few points to consider:

• example 1 - all strings use double quotes;
• example 2 - no strings (except the last two) use quotes;
• the YAML cookbook says: Enclosing strings in double quotes allows you to use escaping to represent ASCII and Unicode characters. Does this mean I need to use double quotes only when I want to escape some characters? If yes - why do they use double quotes everywhere in the first example - only for the sake of unity / stylistic reasons?
• the last two lines of example 2 use ! - the non-specific tag, while the last two lines of the first example don't - and they both work.

My question is: what are the rules for using the different types of quotes in YAML?

Could it be said that:

• in general, you don't need quotes;
• if you want to escape characters use double quotes;
• use ! with single quotes, when... ?!?

I had this concern when working on a Rails application with Docker.

My most preferred approach is to generally not use quotes. This includes not using quotes for:

• variables like ${RAILS_ENV}
• values separated by a colon (:) like postgres-log:/var/log/postgresql
• other strings values

I, however, use double-quotes for integer values that need to be converted to strings like:

• docker-compose version like version: "3.8"
• port numbers like "8080:8080"

However, for special cases like booleans, floats, integers, and other cases, where using double-quotes for the entry values could be interpreted as strings, please do not use double-quotes.

Here's a sample docker-compose.yml file to explain this concept:

version: "3"

services:
  traefik:
    image: traefik:v2.2.1
    command:
      - --api.insecure=true # Don't do that in production
      - --providers.docker=true
      - --providers.docker.exposedbydefault=false
      - --entrypoints.web.address=:80
    ports:
      - "80:80"
      - "8080:8080"
    volumes:
      - /var/run/docker.sock:/var/run/docker.sock:ro

That's all.

I hope this helps