Proposal: Build to Docker Daemon (#223)

proposals/docker_build.md

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+# Proposal: Build to Docker Daemon
+
+## Motivation
+
+Currently, Jib builds and pushes container images to a Docker registry without the need for a Docker daemon. However, for development use cases where the developer does not have a registry set up but do have a Docker daemon available, they may wish to build to the Docker daemon directly.
+
+## Goals
+
+* Build to local Docker daemon
+* Build to [minikube](https://github.com/kubernetes/minikube) (remote) Docker daemon
+* Minimize extra configuration
+
+## Constraints
+
+* Building to local Docker daemon should be an extra feature, not the default
+
+## Intended Workflow
+
+Building to a Docker daemon should be as simple as calling a new task/goal (`jib:buildDocker` for Maven and `jibBuildDocker` for Gradle).
+
+## Implementation
+
+There are three main parts to implementing this proposal:
+
+1. Build an image tarball stream, with the format (see [tarexport/load.go](https://github.com/moby/moby/blob/master/image/tarexport/load.go) for full details):
+  - Each layer as a GZipped tarball named with its SHA256 digest.
+  - A `config.json` as the container config, containing:
+    - `OS`: `linux`
+    - `RootFS.DiffIDS`: in-order [diff IDs](https://github.com/opencontainers/image-spec/blob/master/config.md#layer-diffid) for all the uncompressed layers
+  - A `manifest.json` containing the following fields:
+    - `Config`: The filename of the container config JSON
+    - `Layers`: in-order list of filenames for each layer
+2. Send the image tarball stream to the Docker daemon using [`docker load`](https://docs.docker.com/engine/reference/commandline/load/) CLI command.
+
+Note that currently, build, cache, and push are all in `BuildImageSteps`. In order to implement parts 2 and 3, these stages would need be modularized so that push can be replaced with build image tarball stream and send to Docker daemon.
+
+If in the future, we would like to not use the `docker` CLI, we would need to:
+
+1. Find the Docker daemon.
+  - This should use the same Docker daemon that the `docker` CLI would.
+    - The default host is `unix:///var/run/docker.sock` or `tcp://127.0.0.1:2375`.
+  - Remote Docker daemon can be configured with environment variables, such as those output by `minikube docker-env`. These include:
+    - `DOCKER_TLS_VERIFY`
+    - `DOCKER_HOST`
+    - `DOCKER_CERT_PATH`
+    - `DOCKER_API_VERSION`
+2. Send the image tarball stream to the Docker daemon using the [Docker Engine API](https://docs.docker.com/engine/api/v1.37/#operation/ImageLoad). This can be done manually with HTTP requests, or by using a Docker-Java client like [docker-java](https://github.com/docker-java/docker-java).