is there an actual problem this pr is supposed to address? SSL types handled by go-flags library and won't allow to pass anything except what defined in choice tags

Type string long:"type" env:"TYPE" description:"ssl (auto) support" choice:"none" choice:"static" choice:"auto" default:"none"`

I made this PR after I mistyped SSL_TYPE=true a couple of times myself and didn't get any error.
I believe returning an error in this case will not degrade the user experience. But it can help in finding an erroneous configuration

ah, you meant from env, because this choice handles command-line flags only I think. makes sense

go run app/main.go --dbg --ssl.type=true    
reproxy unknown
Invalid value `true' for option `--ssl.type'. Allowed values are: none, static or auto
2022/02/15 02:49:00 ERROR cli error: Invalid value `true' for option `--ssl.type'. Allowed values are: none, static or auto

could you please match the error your return to the one cli will throw?, just for consistency, i.e.

Invalid value "true" for SSL_TYPE. Allowed values are: none, static or auto

LGTM, will merge.

the minor thing just FYI: you don't need to put \"%s\", this is what %q is for. I'll change it after merge