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.h

@@ -80,6 +80,7 @@ public:
     };
 
     void setEnvironment(const Environment &env);
+    void unsetEnvironment();
     const Environment &environment() const;
 
     void setCommand(const CommandLine &cmdLine);