For Node-based applications, nodemon is a great tool to watch for
file changes and then restart the application. There are equivalent
tools in most other languages and frameworks.
While using env vars to set connection settings is generally ok for development, it is HIGHLY DISCOURAGED when running applications in production. Diogo Monica, a former lead of security at Docker, wrote a fantastic blog post explaining why.
A more secure mechanism is to use the secret support provided by
your container orchestration framework. In most cases, these secrets
are mounted as files in the running container. You'll see many apps
(including the MySQL image and the todo app) also support env vars
with a _FILE suffix to point to a file containing the variable.
As an example, setting the MYSQL_PASSWORD_FILE var will cause the
app to use the contents of the referenced file as the connection
password. Docker does not do anything to support these env vars. Your
app will need to know to look for the variable and get the file
contents.