The characters allowed to represent a container name is represented by the regex : src code

// RestrictedNameChars collects the characters allowed to represent a name, normally used to validate container and volume names.
const RestrictedNameChars = `[a-zA-Z0-9][a-zA-Z0-9_.-]`

see #3138

And only the images name has a limited size of 30 chars.

Container names must start with an alphanumeric character and can then use _ . or - in addition to alphanumeric. [a-zA-Z0-9][a-zA-Z0-9_.-]+

I'm not sure there is a hard limit set for the length of a container name. You might run into some HTTP URL limits at some point as the name ends up in the path for a number of API calls.