Dockerfile: add test stage

Signed-off-by: CrazyMax <crazy-max@users.noreply.github.com>

docker-bake.hcl

@@ -1,8 +1,14 @@
 variable "GO_VERSION" {
   default = "1.16.7"
 }
+
+# Defines the output folder
 variable "DESTDIR" {
-  default = "./bin"
+  default = ""
+}
+function "bindir" {
+  params = [defaultdir]
+  result = DESTDIR != "" ? DESTDIR : "./bin/${defaultdir}"
 }
 
 target "_common" {
@@ -15,10 +21,16 @@ group "default" {
   targets = ["binaries"]
 }
 
+target "test" {
+  inherits = ["_common"]
+  target = "test-coverage"
+  output = [bindir("coverage")]
+}
+
 target "binaries" {
   inherits = ["_common"]
   target = "binaries"
-  output = [DESTDIR]
+  output = [bindir("build")]
   platforms = [
     "darwin/amd64",
     "darwin/arm64",