Version is passed via the tag at build time, so the Dockerfile
default was misleading. The justfile verifies the upstream image
exists before pushing the tag.

Assisted-by: Claude Opus 4.6 via pi

diff --git a/Dockerfile b/Dockerfile
index 52ae575..316a47e 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -1,4 +1,4 @@
-ARG CADDY_VERSION="2.9.1"
+ARG CADDY_VERSION
 
 FROM caddy:${CADDY_VERSION}-builder AS builder
 
diff --git a/justfile b/justfile
new file mode 100644
index 0000000..412200e
--- /dev/null
+++ b/justfile
@@ -0,0 +1,3 @@
+release version:
+    docker manifest inspect caddy:{{version}} > /dev/null
+    jj git push --named v{{version}}=@-