(keyword-lambda ((offset))
   (+ base (or offset 0)))

expands to

(with-let (unlet)
   (lambda* ((offset #f))
     (+ base (or offset 0))))

where lambda* et al are from the built-in startup global environment,
so the local variable is happy:

(define xyz
   (let ((base 123))
     (keyword-lambda ((offset))
       (+ base (or offset 0)))))

(writeln (xyz)) ; 123
(writeln (keyword-call xyz (offset 3))) ;126

(with-let (unlet) ...) goes underneath the global environment, so to
speak,
giving you the built-in startup values, so even set! at the top level
won't
affect it.  There are, of course, many other choices.  I added some more
examples to s7.html this morning.

I hate to admit I have no idea what docker is -- I'll check it out.