Load the env file into the app and worker containers

[chetanr25]

Load the env file into the app and worker containers

Settings in the env file never reached the running code. A new setting looked like it did nothing, and nothing pointed at why.

Why

• Compose reads the env file through --env-file. That only fills in the ${} placeholders inside the compose file itself.
• Nothing from it lands in the container. Every value had to be repeated by hand in the environment block.
• Miss that second line and the setting is silently absent. No error, no warning.

What changed

• docker/dev/compose.yml: env_file on the app and worker services.
• docker/prod/compose.yml: same on the app service.
• docker/README.md: what env_file does, and how it differs from --env-file.

Part of #674