ci: improve docker caching

Dockerfile

@@ -1,4 +1,9 @@
-FROM golang:1.19.4 as build
+FROM golang:1.19.4 as modules
+WORKDIR /build
+ADD go.* /build
+RUN go mod download
+
+FROM modules as build
 WORKDIR /build
 ENV CGO_ENABLED=0
 ADD . /build