Document new masking requirements

app/models/concerns/maskable.rb

@@ -7,7 +7,8 @@ module Maskable
   # * No escape characters
   # * No variables
   # * No spaces
-  # * Minimal length of 8 characters from the Base64 alphabets (RFC4648)
+  # * Minimal length of 8 characters
+  # * Characters must be from the Base64 alphabet (RFC4648) with the addition of @ and :
   # * Absolutely no fun is allowed
   REGEX = /\A[a-zA-Z0-9_+=\/@:-]{8,}\z/.freeze

doc/ci/variables/README.md

@@ -94,7 +94,7 @@ This means that the value of the variable will be hidden in job logs,
 though it must match certain requirements to do so:
 - The value must be in a single line.
-- The value must only consist of characters from the Base64 alphabet, defined in [RFC4648](https://tools.ietf.org/html/rfc4648).
+- The value must only consist of characters from the Base64 alphabet ([RFC4648](https://tools.ietf.org/html/rfc4648)) with the addition of `@` and `:`.
 - The value must be at least 8 characters long.
 - The value must not use variables.