Don't add -u flag to local remote worker docker command on Windows.

-u doesn't currently make sense for Windows: https://github.com/docker/for-win/issues/636#issuecomment-293653788

The local remote worker happens to sometimes work on Windows because we would frequently (always?) hit the timeout here (but recently this hasn't been the case for me): https://github.com/bazelbuild/bazel/blob/fa36d2f48965b127e8fd397348d16e991135bfb6/src/tools/remote/src/main/java/com/google/devtools/build/remote/worker/ExecutionServer.java#L323

When we don't hit the timeout (so "id -u" succeeds, and the "-u" flag is passed to docker), we get an error like this:

```
Error response from daemon: container 06851b64e09bab5a930bfb706892785b24c7538c1a7be826fef315ab8e62c117 encountered an error during CreateProcess: failure in a Windows system call: The user name or password is incorrect. (0x52e)
```

The method for detecting Windows is the best I could find, similar to other isWindows functions in Bazel.

RELNOTES: None.
PiperOrigin-RevId: 193666199

1 files changed, 13 insertions, 4 deletions

diff --git a/src/tools/remote/src/main/java/com/google/devtools/build/remote/worker/ExecutionServer.java b/src/tools/remote/src/main/java/com/google/devtools/build/remote/worker/ExecutionServer.java
index 098b4adf4b..e9ea69df1d 100644
--- a/src/tools/remote/src/main/java/com/google/devtools/build/remote/worker/ExecutionServer.java
+++ b/src/tools/remote/src/main/java/com/google/devtools/build/remote/worker/ExecutionServer.java
@@ -288,6 +288,11 @@ final class ExecutionServer extends ExecutionImplBase {
     return finalResult;
   }
 
+  // Returns true if the OS being run on is Windows (or some close approximation thereof).
+  private boolean isWindows() {
+    return System.getProperty("os.name").startsWith("Windows");
+  }
+
   private boolean wasTimeout(long timeoutMillis, long wallTimeMillis) {
     return timeoutMillis > 0 && wallTimeMillis > timeoutMillis;
   }
@@ -370,10 +375,14 @@ final class ExecutionServer extends ExecutionImplBase {
       newCommandLineElements.add("docker");
       newCommandLineElements.add("run");
 
-      long uid = getUid();
-      if (uid >= 0) {
-        newCommandLineElements.add("-u");
-        newCommandLineElements.add(Long.toString(uid));
+      // -u doesn't currently make sense for Windows:
+      // https://github.com/docker/for-win/issues/636#issuecomment-293653788
+      if (!isWindows()) {
+        long uid = getUid();
+        if (uid >= 0) {
+          newCommandLineElements.add("-u");
+          newCommandLineElements.add(Long.toString(uid));
+        }
       }
 
       String dockerPathString = pathString + "-docker";