Update code to use the "docker compose" syntax vice "docker-compose"

The "docker compose" syntax is the preferred (and only correct) syntax
after the changes in cisagov/ansible-role-docker#60.

README.md

@@ -55,7 +55,7 @@ docker run cisagov/gatherer:1.5.5
 1. Start the container and detach:
 
     ```console
-    docker-compose up --detach
+    docker compose up --detach
     ```
 
 ## Using secrets with your container ##
@@ -108,13 +108,13 @@ environment variables.  See the
 1. Pull the new image from Docker Hub:
 
     ```console
-    docker-compose pull
+    docker compose pull
     ```
 
 1. Recreate the running container by following the [previous instructions](#running-with-docker-compose):
 
     ```console
-    docker-compose up --detach
+    docker compose up --detach
     ```
 
 ### Docker ###

docker-compose.yml

@@ -1,7 +1,7 @@
 ---
 version: "3.7"
 
-# This docker-compose file is used to build the container
+# This docker compose file is used to build and test the container
 
 secrets:
   database_creds:

tests/container_test.py

@@ -11,7 +11,7 @@
 # import pytest
 
 ENV_VAR = "ECHO_MESSAGE"
-ENV_VAR_VAL = "Hello World from docker-compose!"
+ENV_VAR_VAL = "Hello World from docker compose!"
 READY_MESSAGE = "This is a debug message"
 SECRET_QUOTE = (
     "There are no secrets better kept than the secrets everybody guesses."  # nosec