Update content/reference/compose-file/interpolation.md

Co-authored-by: David Karlsson <[email protected]>

Author: aevesdocker

content/reference/compose-file/interpolation.md

@@ -32,7 +32,7 @@ Other extended shell-style features, such as `${VARIABLE/foo/bar}`, are not
 supported by Compose.
 
 Compose processes any string following a `$` sign as long as it makes it
-a valid variable definition - either an alphanumeric name (`[_a-z][_a-z0-9]*`)
+a valid variable definition - either an alphanumeric name (`[_a-zA-Z][_a-zA-Z0-9]*`)
 or a braced string starting with `${`. In other circumstances, it will be preserved without attempting to interpolate a value.
 
 You can use a `$$` (double-dollar sign) when your configuration needs a literal