Workaround for #33 - go sub-modules

Fixes: #33

Go sub-modules do not work with this template, because the of
limitations on how local imports and the "replace" word works
in Go modules. You can only use "replace" in the root-level module
i.e. "main.go"

https://github.com/golang/go/wiki/Modules#gomod

This patch was tested with the following repo:

[email protected]:alexellis/golang-middleware-relative-import.git

* Documentation in the README has been updated
* Tested e2e and with a local "go test" in GOPATH with modules
turned on and Go 1.13
* Updated copy commands in Dockerfile so that they use a faster
mechanism, vs creating a new container step to chown (must be
tested with buildkit before merging, since buildkit did not like
this approach in the past when running as a restricted user at
runtime)

Signed-off-by: Alex Ellis (OpenFaaS Ltd) <[email protected]>

Author: alexellis

README.md

@@ -299,3 +299,53 @@ func Handle(w http.ResponseWriter, r *http.Request) {
 	w.Write([]byte(result))
 }
 ```
+
+#### Advanced usage - Go sub-modules via `GO_REPLACE.txt`
+
+For this example you will need to be using Go 1.13 or newer and Go modules, enable this via `faas-cli build --build-arg GO111MODULE=on`.
+
+Imagine you have a package which you want to store outside of the `handler.go` file, it's another middleware which can perform an echo.
+
+```Golang
+package handlers
+
+import (
+	"io/ioutil"
+	"net/http"
+)
+
+func Echo(w http.ResponseWriter, r *http.Request) {
+
+	if r.Body != nil {
+		defer r.Body.Close()
+		b, _ := ioutil.ReadAll(r.Body)
+		w.Write(b)
+	}
+
+}
+```
+
+To include a relative module such as this new `handlers` package, you should create a `GO_REPLACE.txt` file as follows.
+
+Let's say your GOPATH for your GitHub repo is: `github.com/alexellis/vault/` and your function is called `purchase`, this makes a total path of: `github.com/alexellis/vault/purchase/`
+
+```
+replace github.com/alexellis/vault/purchase/handlers => ./function/handlers
+```
+
+Now if you want to reference the handlers package from within your `handler.go` write the following:
+
+```golang
+package function
+
+import (
+	"net/http"
+
+	"github.com/alexellis/vault/purchase/handlers"
+)
+
+func Handle(w http.ResponseWriter, r *http.Request) {
+
+	handlers.Echo(w, r)
+}
+```

template/golang-middleware/Dockerfile

@@ -12,14 +12,21 @@ RUN mkdir -p /go/src/handler
 WORKDIR /go/src/handler
 COPY . .
 
+RUN $(cat function/GO_REPLACE.txt >> ./go.mod || exit 0) && cat go.mod
+
 # Run a gofmt and exclude all vendored code.
 RUN test -z "$(gofmt -l $(find . -type f -name '*.go' -not -path "./vendor/*" -not -path "./function/vendor/*"))" || { echo "Run \"gofmt -s -w\" on your Golang code"; exit 1; }
 
 ARG GO111MODULE="off"
 ARG GOPROXY=""
+ARG GOFLAGS=""
+
+WORKDIR /go/src/handler/function
+
+RUN go test ./... -cover
 
+WORKDIR /go/src/handler
 RUN go build --ldflags "-s -w" -a -installsuffix cgo -o handler .
-RUN go test handler/function/... -cover
 
 FROM alpine:3.11
 # Add non root user and certs
@@ -30,11 +37,9 @@ RUN apk --no-cache add ca-certificates \
 
 WORKDIR /home/app
 
-COPY --from=build /go/src/handler/handler    .
-COPY --from=build /usr/bin/fwatchdog         .
-COPY --from=build /go/src/handler/function/  .
-
-RUN chown -R app /home/app
+COPY --from=build --chown=app /go/src/handler/handler    .
+COPY --from=build --chown=app /usr/bin/fwatchdog         .
+COPY --from=build --chown=app /go/src/handler/function/  .
 
 USER app