Remove potential leading colon from PYTHONPATH (#356)

* Remove potential leading colon from PYTHONPATH

* Create `io.py` module in test directory as a regression test

Dockerfile

@@ -190,7 +190,7 @@ COPY --from=build /go/pkg/mod/github.com/mwitkow/go-proto-validators@v${go_mwitk
 # Copy mypy
 COPY --from=build /opt/mypy-protobuf/ /opt/mypy-protobuf/
 RUN mv /opt/mypy-protobuf/bin/* /usr/local/bin/
-ENV PYTHONPATH="${PYTHONPATH}:/opt/mypy-protobuf/"
+ENV PYTHONPATH="${PYTHONPATH:+$PYTHONPATH:}/opt/mypy-protobuf/"
 
 ADD all/entrypoint.sh /usr/local/bin
 RUN chmod +x /usr/local/bin/entrypoint.sh

all/test/all_test.go

@@ -688,6 +688,17 @@ func (s *TestSuite) TestAllCases() {
 				defer os.RemoveAll(dir)
 				s.Require().NoError(err)
 				err = copy.Copy(src, path.Join(dir, "all"), opt)
+
+				// We previously had a buggy PYTHONPATH which would search the working directory for
+				// modules when running the Python generator.
+				//
+				// Let's add something that with the same name as a standard library module to the test
+				// directory as a regression test.
+				//
+				// Further context: https://github.com/namely/docker-protoc/pull/356
+				_, err = os.OpenFile(path.Join(dir, "io.py"), os.O_RDONLY|os.O_CREATE, 0666)
+				s.Require().NoError(err)
+
 				argsStr := fmt.Sprintf(DockerCmd,
 					dir,
 					container,