Add example environment configuration and Docker Compose setup

- Introduced a new `.env.example` file to provide a template for environment variables used by the Scratchbox server.
- Added a `docker-compose.example.yml` file to facilitate easy deployment of the Scratchbox service with Docker.
- Updated `docker-entrypoint.sh` to enforce environment variable settings for server configuration.
- Removed the generation of a config key file from the workflow, simplifying the configuration process.
- Adjusted the README.md to reflect changes in configuration management and usage instructions.

docker-entrypoint.sh

@@ -1,12 +1,24 @@
 #!/bin/sh
 set -eu
 
-# Default to running the server with mounted config.
+# Container-enforced data directory.
+SCRATCHBOX_STORAGE_DATA_DIR=/data
+export SCRATCHBOX_STORAGE_DATA_DIR
+
+# Container-enforced listen address/port.
+SCRATCHBOX_SERVER_LISTEN_ADDR=:8080
+export SCRATCHBOX_SERVER_LISTEN_ADDR
+
+# Container-enforced access log behavior: stdout only (no file sink).
+SCRATCHBOX_SERVER_ACCESS_LOG_FILE_PATH=
+export SCRATCHBOX_SERVER_ACCESS_LOG_FILE_PATH
+
+# Default to running the server from SCRATCHBOX_* env vars.
 if [ "$#" -eq 0 ]; then
-  set -- server --config /config/config.yaml
+  set -- server --env
 elif [ "${1#-}" != "$1" ]; then
-  # Allow passing server flags directly, e.g. `docker run ... --config ...`.
-  set -- server "$@"
+  # Allow passing server flags directly, e.g. `docker run ... --env`.
+  set -- server --env "$@"
 fi
 
 exec scratchbox "$@"