fix(docker): Use IMAGE_NAME in api image (#3786)

If you set the IMAGE_NAME variable, then the base image will use that name,
but the api image would previously use a hardcoded `deepset/haystack` image name.

Author: FabianHertwig

docker/Dockerfile.api

@@ -1,6 +1,7 @@
 ARG base_image_tag
+ARG base_image
 
-FROM deepset/haystack:${base_image_tag}
+FROM ${base_image}:${base_image_tag}
 
 ENV SERVICE_NAME="gunicorn-service"
 

docker/docker-bake.hcl

@@ -68,6 +68,7 @@ target "cpu" {
   dockerfile = "Dockerfile.api"
   tags = ["${IMAGE_NAME}:cpu-${IMAGE_TAG_SUFFIX}"]
   args = {
+    base_image = "${IMAGE_NAME}"
     base_image_tag = "base-cpu-${BASE_IMAGE_TAG_SUFFIX}"
   }
   platforms = ["linux/amd64", "linux/arm64"]
@@ -83,6 +84,7 @@ target "gpu" {
   dockerfile = "Dockerfile.api"
   tags = ["${IMAGE_NAME}:gpu-${IMAGE_TAG_SUFFIX}"]
   args = {
+    base_image = "${IMAGE_NAME}"
     base_image_tag = "base-gpu-${BASE_IMAGE_TAG_SUFFIX}"
   }
   platforms = ["linux/amd64", "linux/arm64"]