Fix sample project incorporating comments from @mboes

Full comment at tweag/rules_nixpkgs#39 (comment)

docker_run.sh

@@ -2,5 +2,5 @@
 set -ex
 
 IMAGE=$1
-docker run -v $(pwd)/project:/workdir -w=/workdir --entrypoint="/bin/bash" ${IMAGE} run.sh
+docker run -v $(pwd)/project:/workdir -p 8080:8080 -w=/workdir --entrypoint="/bin/bash" ${IMAGE} run.sh
 

project/.bazelrc

@@ -0,0 +1 @@
+run --python_top=//:python-2.7.15

project/BUILD

@@ -2,8 +2,10 @@
 py_binary(
   name="app",
   srcs=["app.py"],
-  deps = [
-      "@python-2.7.15//:python27Packages.python",
-      "@python2.7-Flask-1.0.2//:python27Packages.flask"
-  ]
+)
+
+py_runtime(
+  name="python-2.7.15",
+  files=[],
+  interpreter="@python-2.7.15//:interpreter",
 )

project/WORKSPACE

@@ -13,13 +13,14 @@ nixpkgs_git_repository(
 
 nixpkgs_package(
     name= "python-2.7.15",
-    attribute_path= "python27Packages.python",
-    repositories = {"@nixpkgs": "nixpkgs"}
+    nix_file= "//:python.nix",
+    repositories = {"nixpkgs": "@nixpkgs//:default.nix"},
+    build_file_content= """
+package(default_visibility = ["//visibility:public"])
+filegroup(
+    name = "interpreter",
+    srcs = ["bin/python"],
 )
-
-nixpkgs_package(
-    name = "python2.7-Flask-1.0.2",
-    attribute_path= "python27Packages.flask",
-    repositories = { "@nixpkgs": "nixpkgs" }
+"""
 )
 

project/app.py

@@ -4,3 +4,10 @@
 @app.route('/')
 def hello():
     return "Hello World!"
+
+if __name__ == '__main__':
+    app.run(
+        host='0.0.0.0',
+        port=8080,
+        debug=True
+    )

project/python.nix

@@ -0,0 +1,3 @@
+with (import <nixpkgs> {});
+
+python27.withPackages (pkgs: [pkgs.flask])

project/run.sh

@@ -3,4 +3,4 @@
 
 set -ex
 
-bazel build :app
+bazel run :app