version: "3.7"
services:
    variables:
        image: busybox
        command: ["/bin/busybox", "sh", "-c", "export | grep EXAMPLE"]
        environment:
            EXAMPLE_VARIABLE: "Host user: $USER"
            EXAMPLE_BRACES: "Host user: ${USER}"
            EXAMPLE_COLON_DASH_DEFAULT: ${NOT_A_VARIABLE:-My default}
            EXAMPLE_DASH_DEFAULT: ${NOT_A_VARIABLE-My other default}
            EXAMPLE_DOT_ENV: $DOT_ENV_VARIABLE
            EXAMPLE_LITERAL: This is a $$literal
            EXAMPLE_EMPTY: $NOT_A_VARIABLE