nesting: document how to enable nesting in container configurations

src/lxc/Makefile.am

@@ -145,6 +145,7 @@ AM_CFLAGS=-I$(top_srcdir)/src \
 	-DLXCINITDIR=\"$(LXCINITDIR)\" \
 	-DLIBEXECDIR=\"$(LIBEXECDIR)\" \
 	-DLXCTEMPLATEDIR=\"$(LXCTEMPLATEDIR)\" \
+	-DLXCTEMPLATECONFIG=\"$(LXCTEMPLATECONFIG)\" \
 	-DLOGPATH=\"$(LOGPATH)\" \
 	-DLXC_DEFAULT_CONFIG=\"$(LXC_DEFAULT_CONFIG)\" \
 	-DLXC_USERNIC_DB=\"$(LXC_USERNIC_DB)\" \

src/lxc/lxccontainer.c

@@ -1375,6 +1375,9 @@ static bool prepend_lxc_header(char *path, const char *t, char *const argv[])
 	fprintf(f, "\n");
 #endif
 	fprintf(f, "# For additional config options, please look at lxc.container.conf(5)\n");
+	fprintf(f, "\n# Uncomment the following line to support nesting containers:\n");
+	fprintf(f, "#lxc.include = " LXCTEMPLATECONFIG "/nesting.conf\n");
+	fprintf(f, "# (Be aware this has security implications)\n\n");
 	if (fwrite(contents, 1, flen, f) != flen) {
 		SYSERROR("Writing original contents");
 		free(contents);