packaging/docker-image: Move Docker files to their own directory

Like other packaging files, they should not be part of RabbitMQ itself.
One day, they will probably be moved to a dedicated repository, like
other packaging files.

Makefile

@@ -340,7 +340,8 @@ clean-unpacked-source-dist:
 .PHONY: packages package-deb \
 	package-rpm package-rpm-fedora package-rpm-suse \
 	package-windows \
-	package-generic-unix
+	package-generic-unix \
+	docker-image
 
 # This variable is exported so sub-make instances know where to find the
 # archive.
@@ -350,28 +351,11 @@ packages package-deb package-rpm \
 package-rpm-redhat package-rpm-fedora package-rpm-rhel6 package-rpm-rhel7 \
 package-rpm-rhel8 package-rpm-suse package-rpm-opensuse package-rpm-sles11 \
 package-windows \
-package-generic-unix: $(PACKAGES_SOURCE_DIST_FILE)
+package-generic-unix \
+docker-image: $(PACKAGES_SOURCE_DIST_FILE)
 	$(verbose) $(MAKE) -C packaging $@ \
 		SOURCE_DIST_FILE=$(abspath $(PACKAGES_SOURCE_DIST_FILE))
 
-$(PACKAGES_DIR)/rabbitmq-server-generic-unix-$(PROJECT_VERSION).tar.xz:
-	$(verbose) $(MAKE) package-generic-unix
-
-# Docker is different because we need to build & publish
-OTP_VERSION := 22.3
-OTP_SHA256 := 886e6dbe1e4823c7e8d9c9c1ba8315075a1a9f7717f5a1eaf3b98345ca6c798e
-docker-image: $(PACKAGES_DIR)/rabbitmq-server-generic-unix-$(PROJECT_VERSION).tar.xz
-	$(verbose) (cd PACKAGES && tar xJvf rabbitmq-server-generic-unix-$(PROJECT_VERSION).tar.xz) \
-	&& docker build --pull \
-	  --build-arg PGP_KEYSERVER=pgpkeys.uk \
-	  --build-arg OTP_VERSION=$(OTP_VERSION) \
-	  --build-arg OTP_SHA256=$(OTP_SHA256) \
-	  --build-arg RABBITMQ_VERSION=$(PROJECT_VERSION) \
-	  --build-arg RABBITMQ_BUILD=PACKAGES/rabbitmq_server-$(PROJECT_VERSION) \
-	  --tag pivotalrabbitmq/rabbitmq:$(PROJECT_VERSION) \
-	  . \
-	&& docker push pivotalrabbitmq/rabbitmq:$(PROJECT_VERSION)
-
 # --------------------------------------------------------------------
 # Installation.
 # --------------------------------------------------------------------

packaging/Makefile

@@ -39,7 +39,8 @@ all: packages
 	package-rpm-rhel6 package-rpm-rhel7 package-rpm-rhel8 \
 	package-rpm-suse package-rpm-opensuse package-rpm-sles11 \
 	package-windows \
-	package-generic-unix
+	package-generic-unix \
+	docker-image
 
 PACKAGES_DIR ?= ../PACKAGES
 SOURCE_DIST_FILE ?= $(wildcard $(PACKAGES_DIR)/rabbitmq-server-*.tar.xz)
@@ -108,9 +109,12 @@ package-windows: $(SOURCE_DIST_FILE)
 package-generic-unix: $(SOURCE_DIST_FILE)
 	$(gen_verbose) $(MAKE) -C generic-unix $(VARS) all $(DO_CLEAN)
 
+docker-image: package-generic-unix
+	$(gen_verbose) $(MAKE) -C docker-image $(VARS) all $(DO_CLEAN)
+
 .PHONY: clean
 
 clean:
-	for subdir in debs/Debian RPMS/Fedora windows windows-exe generic-unix; do \
+	for subdir in debs/Debian RPMS/Fedora windows windows-exe generic-unix docker-image; do \
 		$(MAKE) -C "$$subdir" clean; \
 	done

packaging/docker-image/Makefile

@@ -0,0 +1,35 @@
+GENERIC_UNIX_ARCHIVE ?= $(wildcard $(PACKAGES_DIR)/rabbitmq-server-generic-unix-*.tar.xz)
+
+ifneq ($(filter-out clean,$(MAKECMDGOALS)),)
+ifeq ($(GENERIC_UNIX_ARCHIVE),)
+$(error Cannot find generic-unix archive; please specify GENERIC_UNIX_ARCHIVE)
+endif
+ifneq ($(words $(SOURCE_DIST_FILE)),1)
+$(error Multiple generic-unix archives found; please specify GENERIC_UNIX_ARCHIVE)
+endif
+
+VERSION ?= $(patsubst rabbitmq-server-generic-unix-%.tar.xz,%,$(notdir $(GENERIC_UNIX_ARCHIVE)))
+ifeq ($(VERSION),)
+$(error Cannot determine version; please specify VERSION)
+endif
+endif
+
+OTP_VERSION ?= 22.3
+OTP_SHA256 ?= 886e6dbe1e4823c7e8d9c9c1ba8315075a1a9f7717f5a1eaf3b98345ca6c798e
+
+all: dist
+
+dist:
+	xzcat $(GENERIC_UNIX_ARCHIVE) | tar xvf -
+	echo docker build --pull \
+	  --build-arg PGP_KEYSERVER=pgpkeys.uk \
+	  --build-arg OTP_VERSION=$(OTP_VERSION) \
+	  --build-arg OTP_SHA256=$(OTP_SHA256) \
+	  --build-arg RABBITMQ_VERSION=$(VERSION) \
+	  --build-arg RABBITMQ_BUILD=rabbitmq_server-$(VERSION) \
+	  --tag pivotalrabbitmq/rabbitmq:$(VERSION) \
+	  . \
+	&& docker push pivotalrabbitmq/rabbitmq:$(VERSION)
+
+clean:
+	rm -rf rabbitmq_server-*