Add configurable stratum port (default 3333)

Exposes stratum-port in the StartOS config UI so users running
simpleproxy or another TCP forwarder can point Kamado at a
non-default port. Entrypoint substitutes the value into
ckpool.conf serverurl.

Also pins placeholder KAMADO_SHA in the Dockerfile — swap to a
real pushed commit before the first build.

Dockerfile

@@ -11,7 +11,7 @@
 # WILL fail until the KamadoPool source repo is pushed to GitHub.
 
 ARG KAMADO_REPO=https://github.com/Relaxo143/KamadoPool.git
-ARG KAMADO_SHA=REPLACE_ME_WITH_PUSHED_COMMIT_SHA
+ARG KAMADO_SHA=0000000000000000000000000000000000000000
 ARG ARCH
 
 # ---------- stage 1: fetch source ----------