Docker: Correct environment for docker exec calls

Provide way to clean QtcProcess environment which is needed as
e.g. toolchains, build tools, and debugger otherwise run
inside a wrong environment when using docker exec for the
respective process calls.

Change-Id: Ia6de1c90f2134fce260d293a2f6937ab3cfca2ce
Reviewed-by: hjk <hjk@qt.io>

src/libs/utils/qtcprocess.cpp

@@ -259,6 +259,12 @@ void QtcProcess::setEnvironment(const Environment &env)
     d->m_haveEnv = true;
 }
 
+void QtcProcess::unsetEnvironment()
+{
+    d->m_environment = Environment();
+    d->m_haveEnv = false;
+}
+
 const Environment &QtcProcess::environment() const
 {
     return d->m_environment;