The POSIX standard restricts environment variable names to uppercase
letters, digits, and the underscore character in shell contexts only.
For generic application usage, it is stated that all other characters
shall be tolerated.

This change relaxes the rules to some degree. Namely, we stop requiring
environment variable names to be strict C_IDENTIFIERS and start
permitting lowercase, dot, and dash characters.

Public container images using environment variable names beyond the
shell-only context can benefit from this relaxation. Elasticsearch is
one popular example.