Add help for Makefile

[lucacome]

Adds help (as default target) in the Makefile:

❯ make help
Usage:

    make <target>

Targets:

    build                          Build the binary
    clean-go-cache                 Clean go cache
    clean                          Clean the build
    container                      Build the container
    create-kind-cluster            Create a kind cluster
    delete-kind-cluster            Delete kind cluster
    deps                           Add missing and remove unused modules, verify deps and download them to local cache
    dev-all                        Run all the development checks
    fmt                            Run go fmt against code
    generate                       Run go generate
    help                           Display this help
    lint                           Run golangci-lint against code
    njs-fmt                        Run prettier against the njs httpmatches module
    njs-unit-test                  Run unit tests for the njs httpmatches module
    unit-test                      Run unit tests for the go code
    update-codegen                 Update codegen
    update-crds                    Update CRDs
    verify-codegen                 Verify code generation
    vet                            Run go vet against code

Adds a couple of checks for docker and go

[pleshakov]

👍 nice