Restrict allowed characters in container names

[aanand]

While it’s cool that I can start a container named ‘☃’, ‘▀ ▁ ▂ ▃ ▄ ▅ ▆ ▇ █ ▉ ▊ ▋ ▌ ▍ ▎ ▏ ▐ ░ ▒ ▓ ▔ ▕’ or ‘ ＂ ＃ ＄ ％ ＆ ＇ （ ） ＊ ＋ ， － ． ／ ０ １ ２ ３ ４ ５ ６ ７ ８ ９ ： ； ＜ ＝ ＞ ？ ＠ Ａ Ｂ Ｃ Ｄ Ｅ Ｆ Ｇ Ｈ Ｉ Ｊ Ｋ Ｌ Ｍ Ｎ Ｏ Ｐ Ｑ Ｒ Ｓ Ｔ Ｕ Ｖ Ｗ Ｘ Ｙ Ｚ ［ ＼ ］ ＾ ＿ ｀ ａ ｂ ｃ ｄ ｅ ｆ ｇ ｈ ｉ ｊ ｋ ｌ ｍ ｎ ｏ ｐ ｑ ｒ ｓ ｔ ｕ ｖ ｗ ｘ ｙ ｚ ｛ ｜ ｝ ～ ｡ ｢ ｣ ､ ･’, I'm not convinced it's adding that much value.

After five minutes messing around, / is the one character I've found which does actually cause an error (presumably because it's used as a path separator), so that should definitely be disallowed.

Beyond that, I don't necessarily think Docker should go as far as to only allow, say, a-zA-Z0-9_ (which feels arbitrarily Western-centric, among other things), but the current level of permissiveness feels to me like a bunch of errors waiting to happen.

[crosbymichael]

@aanand What would you like to see added to a-zA-Z0-9_ ?

[bfirsh]

I quite like the idea of app name restrictions being consistent with images and users (i.e. a-zA-Z0-9_).

[jamtur01]

What @bfirsh said seems reasonable to me. +1.

[aanand]

OK, if there's already a standard restriction in place for images and users, doing the same for container names sounds sensible.