Add support for max layer caching in build

[dh-seanmurphy]

With the change to buildkit as the default option, layers which are not required for the final image are not cached.

This PR support caching of all images in a build process, specifically by leveraging the containerd image store and the flag mode=max for the cache-to.

It requires:

• addition of a flag to support enabling the containerd image store
• addition of a CACHE_TO flag
• addition of a CACHE_FROM_EXPLICIT flag which can take an image name of the form type=registry,ref=<registry>/<cache-image>:<branch>

Build using the standard instructions.

Testing:

• main.go

package main

import (
        "fmt"
        "log"
        "net/http"
)

func handler(w http.ResponseWriter, r *http.Request) {
        fmt.Fprint(w, "Hello from Go Docker multistage")
}

func main() {
        http.HandleFunc("/", handler)
        fmt.Println("server running at 0.0.0.0:8080")
        log.Fatal(http.ListenAndServe("0.0.0.0:8080", nil))
}

• Dockerfile

FROM golang:alpine AS builder
WORKDIR /src/app
COPY go.mod main.go ./
RUN go build -o server

FROM alpine
WORKDIR /root/
COPY --from=builder /src/app ./app
CMD ["./app/server"]

• build script

#! /usr/bin/env bash

docker run --rm \
  -e PLUGIN_TAG=latest \
  -e PLUGIN_REPO=proxy-it-apps-harbor.localhost/test-project/go-project \
  -e PLUGIN_CACHE_FROM_EXPLICIT="type=registry,ref=proxy-it-apps-harbor.localhost/test-project/dependencies" \
  -e PLUGIN_CACHE_TO=type=registry,image-manifest=true,oci-mediatypes=true,ref=proxy-it-apps-harbor.localhost/test-project/dependencies,mode=max \
  -e DOCKER_REGISTRY=proxy-it-apps-harbor.localhost \
  -e DOCKER_USERNAME=admin \
  -e DOCKER_PASSWORD=<REDACTED> \
  -v $(pwd):$(pwd) \
  -w $(pwd) \
  --privileged \
  --net=host \
  plugins/docker

• requires a registry to post the images to
• run go mod init
• build with mode=min and mode=max and you can see the different sizes of content pushed to the repository