build(webdav): 添加支持webdav的caddy dockerfile

添加多架构构建的dockerfile，用于构建支持webdav插件的caddy镜像

webout/webdav/dockerfile

@@ -0,0 +1,9 @@
+# RUN: docker buildx build --push --platform linux/amd64,linux/arm64 -t cnphpbb/caddy:2-webdav-alpine .
+FROM caddy:2-builder-alpine AS builder
+
+RUN xcaddy build \
+    --with github.com/abiosoft/caddy-webdav
+
+FROM caddy:2-alpine
+
+COPY --from=builder /usr/bin/caddy /usr/bin/caddy