fn: support for dotnet (#326)

* Add initial support for dotnet

* Initial work on dotnet example

* fn: fix docker incantation

* fn: .gitignore

* Add README.md for dotnet example

* Update docs

fn/langs/base.go

@@ -15,6 +15,8 @@ func GetLangHelper(lang string) (LangHelper, error) {
 		return &PythonHelper{}, nil
 	case "rust":
 		return &RustLangHelper{}, nil
+	case "dotnet":
+		return &DotNetLangHelper{}, nil
 	}
 	return nil, fmt.Errorf("No language helper found for %v", lang)
 }

fn/langs/dotnet.go

@@ -0,0 +1,42 @@
+package langs
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+)
+
+type DotNetLangHelper struct{}
+
+func (lh *DotNetLangHelper) Entrypoint() string {
+	return "dotnet dotnet.dll"
+}
+
+func (lh *DotNetLangHelper) HasPreBuild() bool {
+	return true
+}
+
+// PreBuild for Go builds the binary so the final image can be as small as possible
+func (lh *DotNetLangHelper) PreBuild() error {
+	wd, err := os.Getwd()
+	if err != nil {
+		return err
+	}
+
+	cmd := exec.Command(
+		"docker", "run",
+		"--rm", "-v",
+		wd+":/dotnet", "-w", "/dotnet", "microsoft/dotnet",
+		"/bin/sh", "-c", "dotnet restore && dotnet publish -c release -b /tmp -o .",
+	)
+	cmd.Stderr = os.Stderr
+	cmd.Stdout = os.Stdout
+	if err := cmd.Run(); err != nil {
+		return fmt.Errorf("error running docker build: %v", err)
+	}
+	return nil
+}
+
+func (lh *DotNetLangHelper) AfterBuild() error {
+	return nil
+}