Update documentation and CLI tool help

Author: pont-us

docs/xcetool.md

@@ -26,8 +26,10 @@ Options:
 -   `-e`, `--environment` `FILE`:
     Conda environment file to use in Docker image.
     If no environment file is specified here or in the notebook,
+    xcetool will look for a file called `environment.yml`
+    in the notebook's directory. If all else fails,
     xcetool will try to reproduce the current environment
-    as a last resort, but this is not recommended.
+    as a last resort, but this is not guaranteed to succeed.
 -   `-t`, `--tag` `TEXT`: Tag to apply to the Docker image.
     If not specified, a
     timestamp-based tag will be generated automatically.

xcengine/cli.py

@@ -117,7 +117,9 @@ def image_cli():
     "--environment",
     type=click.Path(path_type=pathlib.Path, dir_okay=False, file_okay=True),
     help="Conda environment file to use in Docker image. "
-    "If not specified, try to reproduce the current environment.",
+    "If no environment file is specified here or in the notebook, and if "
+    "there is no file named environment.yml in the notebook's directory, "
+    "xcetool will try to reproduce the current environment."
 )
 @click.option(
     "-t",