build: Expand build dirs in Makefile.buildkit

Remove support for user-defined build dir paths when using Docker
buildkit. This was dangerous as there was no easy way to make sure the
user supplied paths were pointing to directories that can be safely
deleted and overwritten.

Expand the BUILD_DIR and DOCKER_BUILD_DIR in Makefile.buildkit to make
it easier to understand and to protect against accidental damage when
BUILD_DIR = '.', which would mess up user's main git repo.

Fixes: #11513
Signed-off-by: Jarno Rajahalme <jarno@covalent.io>

Makefile.buildkit

@@ -8,7 +8,7 @@ export DOCKER_BUILDKIT=1
 
 # Clean the build directory and cache
 clean-build:
-	-$(QUIET) rm -rf $(BUILD_DIR)
+	-$(QUIET) rm -rf _build
 	-$(QUIET) docker builder prune --filter type=exec.cachemount -f
 
 veryclean: clean-build
@@ -22,18 +22,18 @@ HUBBLE_RELAY_DOCKERFILE := $(BUILD_DIR)/hubble-relay.Dockerfile
 BUILDKIT_DOCKERFILE_FILTER := | sed -e "1s|^\#.*|\# syntax = docker/dockerfile:experimental|" -e "s|^RUN\(.*\)make|RUN --mount=type=cache,target=/root/.cache/go-build\1make|"
 
 # _build/.git is a shallow bare clone of the main repo
-$(BUILD_DIR)/.git:
-	-mkdir -p $(dir $@)
-	git clone --bare --no-local --depth 1 . $@
-	@cd $(dir $@) && git remote set-url origin "$(shell realpath --relative-to $(dir $@) $(ROOT_DIR))"
+_build/.git:
+	-mkdir -p _build
+	git clone --bare --no-local --depth 1 . _build/.git
+	@cd _build && git remote set-url origin ..
 
 #
 # Create _build context:
-# $(DOCKER_BUILD_DIR)/.git will be a file (rather than directory) that contains git specific
+# _build/context/.git will be a file (rather than directory) that contains git specific
 # symbolic link to _build/.git (--separate-git-dir)
 #
-$(DOCKER_BUILD_DIR): $(BUILD_DIR)/.git
-	git init --separate-git-dir=$(BUILD_DIR)/.git $@
+_build/context: _build/.git
+	git init --separate-git-dir=$< $@
 
 #
 # Update the docker build context by:
@@ -48,19 +48,19 @@ $(DOCKER_BUILD_DIR): $(BUILD_DIR)/.git
 # - update GIT_VERSION in the build context if needed
 # - update BPF_SRCFILES in the build context if needed
 #
-build-context-update: check-status $(DOCKER_BUILD_DIR) $(DOCKER_BUILD_DIR)/GIT_VERSION $(DOCKER_BUILD_DIR)/BPF_SRCFILES
-	@echo "gitdir: ../.git" > $(DOCKER_BUILD_DIR)/.git
-	cd $(BUILD_DIR) && git fetch --depth=1 --no-tags
-	cd $(DOCKER_BUILD_DIR) && git reset --hard FETCH_HEAD && git clean -fd
-	@rm $(DOCKER_BUILD_DIR)/.git
+build-context-update: check-status _build/context _build/context/GIT_VERSION _build/context/BPF_SRCFILES
+	@echo "gitdir: ../.git" > _build/context/.git
+	cd _build && git fetch --depth=1 --no-tags
+	cd _build/context && git reset --hard FETCH_HEAD && git clean -fd
+	@rm _build/context/.git
 
 #
 # Docker build context does not contain the actual git repo, so we need to pass
 # GIT_VERSION and/or BPF_SRCFILES to the build context separately.
 #
-$(DOCKER_BUILD_DIR)/GIT_VERSION: GIT_VERSION $(DOCKER_BUILD_DIR)
+_build/context/GIT_VERSION: GIT_VERSION _build/context
 	cp $< $@
-$(DOCKER_BUILD_DIR)/BPF_SRCFILES: BPF_SRCFILES $(DOCKER_BUILD_DIR)
+_build/context/BPF_SRCFILES: BPF_SRCFILES _build/context
 	cp $< $@
 
 # Bare 'Dockerfile' needs a special rule