An example maintaining a cache in an image

docker_buildkit_cach_test.sh

#!/usr/bin/env bash

set -euo pipefail
IFS=$'\n\t'

function clear_build_cache {
  docker builder prune -f > /dev/null
}

function clean_start {
  rm -f appended.txt
  rm -f Dockerfile
  docker image rm -f cache_image app > /dev/null
  clear_build_cache
}

function bootstrap_cache {
  docker pull bash
  docker tag bash cache_image
}

function create_docker_file {
# shellcheck disable=SC2016
# shellcheck disable=SC1004
  echo \
'FROM cache_image as builder
ARG TO_APPEND=1
RUN mkdir -p /cached_dir && \
    touch /cached_dir/appended.txt && \
    echo $TO_APPEND >> /cached_dir/appended.txt && \
    mkdir -p /artifact_dir && \
    cp /cached_dir/appended.txt /artifact_dir/appended.txt

FROM cache_image as cache
COPY --from=builder /cached_dir /cached_dir

FROM bash as app
COPY --from=builder /artifact_dir/appended.txt /artifact_dir/appended.txt
' > ./Dockerfile
}

function docker_build {
  local to_append=$1
#  export DOCKER_BUILDKIT=1
#  export BUILDKIT_PROGRESS=plain
  docker build \
    . \
    -t app \
    --build-arg TO_APPEND="$to_append"
  docker build \
    . \
    --target cache \
    -t cache_image \
    --build-arg TO_APPEND="$to_append"
}


function assert_results {
  local expected=$1

  local id
  id=$(docker create app)
  docker cp "$id":/artifact_dir/appended.txt appended.txt
  docker rm -v "$id" > /dev/null

  results=$(cat appended.txt)
  echo "$results"
  total_lines="$(wc -l < appended.txt | tr -d ' ')"
  if [[ "$total_lines" != "$expected" ]]; then
    >&2 echo "Expected $expected lines, got $total_lines!"
    exit 1;
  fi
}

function main {
  clean_start
  bootstrap_cache

  create_docker_file ''

  echo
  echo "Build 1"
  docker_build "$(date "+%s")"
  echo

  echo
  echo "Build 2"
  docker_build "$(date "+%s")"
  echo

  echo "After 2 runs, no buildkit cache clear, there should be 2 lines in the file"
  assert_results 2

  clear_build_cache

  echo
  echo "build 3"
  docker_build "$(date "+%s")"
  echo

  echo "After a buildkit cache clear & a further run there should be 3 lines in the file because the previous state should have come from the image cache"
  assert_results 3
}

main