improve cache for labels changes

We have a use case where a dependentant image gets its cache invalidated by the upstream image adding a new label.

We are wondering if there is anything that can be done at buildkit level (since there is so much work done to improve caching) that could help us improve this use case.

We first build a php docker (using docker-compose)

FROM php:7.0-fpm-alpine AS php70

LABEL maintainer "https://github.com/FernandoMiguel"

RUN adduser -DHSu 100 nginx -s /sbin/nologin
COPY --from=composer /usr/bin/composer /usr/bin/composer

COPY ./opcache.ini /usr/local/etc/php/conf.d/opcache.ini

RUN apk add --no-cache --virtual build-dependencies \
    $PHPIZE_DEPS \
    autoconf \
    automake \
    build-base \
    cmake \
    curl-dev \
    file \
    g++ \
    gcc \
    gettext-dev \
    git \
    icu-dev \
    libc-dev \
    libmcrypt-dev \
    libpng-dev \
    libressl-dev \
    libtool \
    libxml2-dev \
    libxslt-dev \
    make \
    nasm \
    pcre-dev \
    pkgconf \
    re2c \
    sqlite-dev \
    wget \
    zlib-dev

[....]

WORKDIR /var/www

docker-compose tags the labels.

version: "3.6"
services:
  php70:
    image: xxx.dkr.ecr.eu-west-1.amazonaws.com/php:release-production-php70
    build: 
      context: ../infrastructure-as-code/Docker/php
      dockerfile: Dockerfile-7.0
      labels:
        COMMIT_ID.KIMI: ${COMMIT_ID_KIMI}
        COMMIT_ID.SELF_SUBMODULE: ${COMMIT_ID_SELF_infrastructure_as_code}

We then build images that use that as the FROM image

ARG PHPV=-php70
FROM XXXX.dkr.ecr.eu-west-1.amazonaws.com/php:release-production${PHPV} AS lumen-builder

LABEL maintainer "https://github.com/FernandoMiguel"

COPY . /src

RUN cd /src && \
	composer -v install --no-dev --no-interaction

Here's the bit from docker-compose

  ms-builder:
    build:
      context: ../ms-xxx
      dockerfile: ../.Docker-lumen/Dockerfile-builder
      args:
        - PHPV=${PHPV}
      labels:
        COMMIT_ID.KIMI: ${COMMIT_ID_KIMI}
        COMMIT_ID.DOCKERFILE_SUBMODULE: ${COMMIT_ID_SELF__Docker_lumen}
    image: ms-builder:release-${PHPV}

Sadly even with no code change in any of these two code bases, if anything changes in the repo, the HEAD will change, making the label be updated.
What happens is that the dependant image will now bust cache cause the upstream image updated the label, and as such the image id.

Start by reproducing the dependent-image cache invalidation described in the Dockerfiles and docker-compose configurations, changing only the upstream labels. Trace how BuildKit handles the upstream image identity and labels, then define and verify behavior where label-only changes do not unnecessarily invalidate dependent builds.