Add a warning about in-container builds

- The binary may not even run on macOS
- It builds an Ubuntu binary which may not run on e.g. Fedora due to
  different library sonames

README.md

@@ -110,6 +110,7 @@ If your system has a `docker` command and can run Linux containers,
 ```sh
 $ make binary # Or (make all) to also build documentation, see below.
 ```
+(Note that the created executable is a Linux executable, and depends on dynamic libraries which may only be available only in a container of a similar Linux distribution.)
 
 ### Building without a container
 Install the necessary dependencies: