docker: Tame xargs warning

From xargs' manual page:

     The options --max-lines (-L, -l), --replace (-I, -i) and --max-args
     (-n) are mutually exclusive. If some of them are specified at the
     same time, then xargs will generally use the option specified last
     on the command line, i.e., it will reset the value of the offending
     option (given before) to its default value. Additionally, xargs
     will issue a warning diagnostic on stderr. The exception to this
     rule is that the special max-args value 1 ('-n1') is ignored after
     the --replace option and its aliases -I and -i, because it would
     not actually conflict.

Just as the manual says, we're getting a warning from Makefile.docker
because we use both -n1 and -I, in this order. The options are not
supposed to conflict in our case, but it so happens that for xargs to
suppress the warning, order matters. It's trivial to tame the warning by
moving the -n1 option _after_ the -I.

We could just as well delete the -n1, but I thought keeping it shows the
original intent of having just one argument per command.

Signed-off-by: Quentin Monnet <[email protected]>

Makefile.docker

@@ -49,7 +49,7 @@ GIT_IGNORE_FILES := $(shell find . -not -path "./vendor*" -name .gitignore -prin
 	@echo "/hack" > $@
 	@echo ".git" >> $@
 	@echo "/Makefile.docker" >> $@
-	@echo $(dir $(GIT_IGNORE_FILES)) | tr ' ' '\n' | xargs -P1 -n1 -I {DIR} sed \
+	@echo $(dir $(GIT_IGNORE_FILES)) | tr ' ' '\n' | xargs -P1 -I {DIR} -n1 sed \
 		-e '# Remove lines with white space, comments and files that must be passed to docker, "$$" due to make. #' \
 			-e '/^[[:space:]]*$$/d' -e '/^#/d' -e '/GIT_VERSION/d' \
 		-e '# Apply pattern in all directories if it contains no "/", keep "!" up front. #' \