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Images containing Go applications

Typical users will migrate from go_image in rules_docker.

Base image

First, you'll need a base image.

It's wise to minimize changes by using the same one your current go_image uses. The logic for choosing the default base in rules_docker is in go_image.bzl

Or, you can just use bazel query to find out:

$ bazel query --output=build hello_world:hello_world_go_image
Starting local Bazel server and connecting to it...
# /home/alexeagle/Projects/hello_world/BUILD.bazel:22:9
_app_layer(
  name = "hello_world_go_image",
  base = select({"@io_bazel_rules_docker//:debug": "@go_debug_image_base//image:image", "@io_bazel_rules_docker//:fastbuild": "@go_image_base//image:image", "@io_bazel_rules_docker//:optimized": "@go_image_base//image:image", "//conditions:default": "@go_image_base//image:image"}),
...
)

Since we don't use the "debug" config, this tells us that we use @go_base_image, we need one more lookup to see what that uses:

$ bazel query --output=build @go_image_base//image:image
# /shared/cache/bazel/user_base/bf7b6accf6f1187bd5511f3fbf7b21b9/external/go_image_base/image/BUILD:4:17
container_import(
  name = "image",
  base_image_registry = "gcr.io",
  base_image_repository = "distroless/base",
...
)

Now that we know it's gcr.io/distroless/base we can pull the same base image by adding to WORKSPACE:

load("@rules_oci//oci:pull.bzl", "oci_pull")

oci_pull(
    name = "distroless_base",
    digest = "sha256:ccaef5ee2f1850270d453fdf700a5392534f8d1a8ca2acda391fbb6a06b81c86",
    image = "gcr.io/distroless/base",
    platforms = ["linux/amd64","linux/arm64"],
)

See more details in the oci_pull docs

The go_image

rules_docker makes you repeat the attributes of go_binary into go_layer. This is a "layering violation" (get it?).

In rules_oci, you just start from a normal go_binary (typically by having Gazelle write it). For this example let's say it's go_binary(name = "app", ...).

Next, put that file into a layer, which is just a .tar file:

load("@rules_pkg//pkg:tar.bzl", "pkg_tar")

pkg_tar(
    name = "tar",
    srcs = [":app"],
)

Finally, add your layer to the base image:

load("@rules_oci//oci:defs.bzl", "oci_image")

oci_image(
    name = "image",
    architecture = select({
        "@platforms//cpu:arm64": "arm64",
        "@platforms//cpu:x86_64": "amd64",
    }),
    base = "@distroless_base",
    tars = [":tar"],
    entrypoint = ["/app"],
    os = "linux",
)

Example

A full example can be found at https://github.com/aspect-build/bazel-examples/tree/main/oci_go_image.