docker-compose local variable declare and use

In the programming language we can define and declare a variable and use it later in the program, can we do same in docker-compose file?

For example in Java:

int x = 100;
System.out.println("Number is: "+ x);

Similarly, in docker-compose file, can we declare a local variables and use it later.

docker-compose file

In the docker-compose file we want to declare tomcat_home as a variable and use it later as shown in example.


values:
- tomcat_home: "/usr/local/tomcat"

# Creating Tomcat Server 1

Server1:
image: tomcat
hostname: TomcatServer1
container_name: TomcatServer1
volumes:
- ${PWD}/tomcat-users.xml:${tomcat_home}/conf/tomcat-users.xml
- ${PWD}/server.xml:${tomcat_home}/conf/server.xml


https://docs.docker.com/compose/environment-variables/#the-env-file

One could set a variable in an .env file:

TAG=1.2.3

and call it in a docker-compose file

docker-compose.yml

version: "3"
services:
  web:
    image: nginx:${TAG}
    ports:
    - 80:80

docker-compose up

returns:

Pulling web (nginx:1.2.3)...
ERROR: manifest for nginx:1.2.3 not found

No need to run docker-compose to check the config

According to the documentation one could run:

docker-compose config

to verify the docker-compose config

networks: {}
services:
  web:
    image: nginx:1.2.3
    ports:
    - 80:80/tcp
version: '3.0'
volumes: {}

StackExchange DevOps Q#4079, answer score: 2