entrypoint.sh: Update help text so that current image name is used
entrypoint.sh: Update help text so that current image name is used

xref https://github.com/dockbuild/dockbuild/issues/30

dockcross/dockcross,dockcross/dockcross,thewtex/cross-compilers,dockcross/dockcross,thewtex/cross-compilers,thewtex/cross-compilers,thewtex/cross-compilers,dockcross/dockcross