fix "Possible nested set" warning
`[:xdigit:]` is not a valid character class in Python regular expressions
use explicit hex digit pattern
MODIFY
repo2docker/utils.py
repo2docker/utils.py
@@ -209,7 +209,7 @@ def is_valid_docker_image_name(image_name):
(?::([\w][\w.-]{0,127}))? # optional capture <tag-pattern>=':<tag>'
# optionally capture <digest-pattern>='@<digest>'
- (?:@[A-Za-z][A-Za-z0-9]*(?:[-_+.][A-Za-z][A-Za-z0-9]*)*[:][[:xdigit:]]{32,})?
+ (?:@[A-Za-z][A-Za-z0-9]*(?:[-_+.][A-Za-z][A-Za-z0-9]*)*[:][A-Fa-f0-9]{32,})?
$
""", re.VERBOSE)

fix "Possible nested set" warning

`[:xdigit:]` is not a valid character class in Python regular expressions

use explicit hex digit pattern