docker: Align also caller's group ID with container

This ensures that both UID and GID of the builder user inside the
container is aligned with the caller of kas-docker - or that of "docker
run" when "-e GROUP_ID=..." is specified.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

docker-entrypoint

@@ -12,6 +12,7 @@ EOF
 fi
 
 USER_ID=${USER_ID:-30000}
+GROUP_ID=${GROUP_ID:-30000}
 
 if [ $USER_ID == 0 ]; then
 	# We shall run everything as root
@@ -19,9 +20,13 @@ if [ $USER_ID == 0 ]; then
 
 	GOSU=""
 else
-	if ! id $USER_ID >/dev/null 2>&1; then
+	if ! grep -q "^builder:" /etc/group; then
+		groupadd -o --gid $GROUP_ID builder
+	fi
+	if ! id builder >/dev/null 2>&1; then
 		# Create a non-root user that will perform the actual build
-		useradd --uid $USER_ID --create-home --home-dir /builder builder
+		useradd -o --uid $USER_ID --gid $GROUP_ID --create-home \
+			--home-dir /builder builder
 	fi
 
 	GOSU="gosu builder"

kas-docker

@@ -128,7 +128,7 @@ trace mkdir -p ${KAS_WORK_DIR}
 
 DOCKER_ARGS="-v ${REPO_DIR}:/repo:ro \
 	     -v ${KAS_WORK_DIR}:/work:rw --workdir=/work \
-	     -e USER_ID=$(id -u) --rm"
+	     -e USER_ID=$(id -u) -e GROUP_ID=$(id -g) --rm"
 
 if [ -t 1 ]; then
 	DOCKER_ARGS="${DOCKER_ARGS} -t -i"