allow override for toolchain docker volume and workdir for niche environments

Makefile

@@ -13,7 +13,7 @@ IMAGE := local/$(NAME)
 UID := $(shell id -u)
 GID := $(shell id -g)
 USER := $(UID):$(GID)
-CPUS := $(shell docker run -it debian nproc)
+CPUS := $(shell docker run debian nproc)
 PRESERVE_CACHE := "false"
 GIT_REF := $(shell git log -1 --format=%H)
 GIT_AUTHOR := $(shell git log -1 --format=%an)
@@ -214,6 +214,8 @@ define fetch_pgp_key
         ')
 endef
 
+TOOLCHAIN_VOLUME := $(PWD):/home/build
+TOOLCHAIN_WORKDIR := /home/build
 define toolchain
         docker run \
                 --rm \
@@ -224,10 +226,11 @@ define toolchain
                 --platform=linux/$(ARCH) \
                 --privileged \
                 --cpus $(CPUS) \
-		--volume $(PWD):/home/build \
-		--workdir /home/build \
+                --volume $(TOOLCHAIN_VOLUME) \
+                --workdir $(TOOLCHAIN_WORKDIR) \
                 --env-file=$(CONFIG_DIR)/global.env \
                 --env-file=$(CACHE_DIR_ROOT)/toolchain.env \
                 $(shell cat cache/toolchain.state 2> /dev/null) \
                 $(SRC_DIR)/toolchain/scripts/host-env bash -c $(1)
 endef
+

scripts/host-env

@@ -6,7 +6,10 @@ gid=${GID?}
 user=${USER:-"build"}
 export HOME="/home/${user}"
 
-groupdel dialout
+# If running user is not root, pivot to custom user/group
+[ "$uid" != "0" ] && {
+    getent group "$gid" && groupdel "$gid"
+    getent passwd "$uid" && userdel "$uid"
     groupadd -g "$gid" "${user}"
     useradd \
         -g "$gid" \
@@ -15,6 +18,7 @@ useradd \
         -d "/home/${user}" \
         -s /bin/bash \
         "${user}"
-
-cd "$HOME"
     setpriv --reuid="$uid" --regid="$gid" --init-groups "$@"
+}
+
+exec "$@"