Fix local docker build of PDF spec

Something changed in the latest docker image that causes asciidoctor-pdf
to fail when run locally (but not when run in CI).  The problem is that
the user is different when running locally, and this user does not have
write access to the HOME directory.  It appears that asciidoctor-pdf
attempts to write temporary files into HOME, so this failed with an
access permission error.

Fix this by setting HOME=/tmp when doing a local docker build.

adoc/Makefile

@@ -268,16 +268,20 @@ clean_pdf:
 #   make docker-clean docker-html docker-pdf
 # Also useful to have a shell inside docker:
 #   make docker-bash
+#
+# Set the user and group inside the container to match the user and group
+# outside of the container.  This causes the generated files to be owned by the
+# current user when accessed from outside the container.  However, this user
+# probably doesn't have write access to the default HOME directory, so set HOME
+# to some directory that does have write access.  This is important because
+# some tools (e.g. asciidoctor-pdf) write temporary files into HOME.  It does
+# not matter that we are polluting /tmp here because the entire container will
+# be discarded when the build is complete.
 docker-%:
-	# Run with current user and group id the published AsciiDoctor
-	# capable Khronos docker image with current SYCL specification
-	# directory mounted in /sycl
-	# Re-set MAKEFLAGS to pass variables to the inner make since
-	# variables are dropped by docker.
-	sudo docker run --user `id --user`:`id --group` \
+	sudo docker run --user `id --user`:`id --group` --env HOME=/tmp \
 	  --interactive --tty --rm \
-          --volume $(SYCL_DIR):/sycl \
-          khronosgroup/docker-images:asciidoctor-spec \
+	  --volume $(SYCL_DIR):/sycl \
+	  khronosgroup/docker-images:asciidoctor-spec \
 	  $(MAKE) MAKEFLAGS="$(MAKEFLAGS)" --directory=/sycl/adoc $*
 
 dock: