Dockerfile: split release and dev recipes (#457)

Combined with the new documentation, this should make it easier to test
Renovate PRs like #455 in the future without changing any actual
behaviour.

Author: LawnGnome

.dockerignore

@@ -0,0 +1,13 @@
+# Anything Git or VSCode related is definitely irrelevant for a build.
+.git*
+.vscode
+
+# The docker directory contains other Docker images.
+docker
+
+# The release directory is created by goreleaser and isn't required here.
+release
+
+# Documentation and examples aren't needed.
+*.md
+*.{yaml,yml}

.goreleaser.yml

@@ -38,6 +38,7 @@ brews:
     # Default is the root folder.
     folder: Formula
 dockers:
+  - dockerfile: Dockerfile.release
   - image_templates:
     - "sourcegraph/src-cli:{{ .Tag }}"
     - "sourcegraph/src-cli:{{ .Major }}"

DEVELOPMENT.md

@@ -39,6 +39,30 @@ If a new feature is added to a new `3.91.6` release of src-cli and this change r
 
 Note that if instead the recommended src-cli version for Sourcegraph `3.99` was `3.90.4` in the example above, there is no additional step required, and the new patch version of src-cli will be available to both Sourcegraph versions.
 
+## `src-cli` Docker image
+
+Each release of `src` results in a new tag of the [`sourcegraph/src-cli` Docker image](https://hub.docker.com/r/sourcegraph/src-cli) being pushed to Docker Hub. This is handled by [goreleaser's Docker support](https://goreleaser.com/customization/docker/).
+
+The main gotcha here is that the way goreleaser builds the Docker image is fairly difficult to replicate from the desktop: it builds a `src` binary without any runtime libc dependencies that can be installed in a `scratch` image, but unless you work on Alpine, your desktop is _not_ configured to build Go binaries like that.
+
+As a result, there are two Dockerfiles in this project. goreleaser uses `Dockerfile.release`, which is replicated in a multi-stage `Dockerfile` that builds `src` in a builder container to ensure it's built in a way that can be tested.
+
+### Testing
+
+If you need to test a change to the Dockerfile (for example, due to a Renovate PR bumping the base image), you should pull that change, then build a local image with something like:
+
+```bash
+docker build -t local-src-cli .
+```
+
+After which you should be able to run:
+
+```bash
+docker run --rm local-src-cli
+```
+
+and get the normal help output from `src`.
+
 ## Dependent Docker images
 
 `src campaign apply` and `src campaign preview` use a Docker image published as `sourcegraph/src-campaign-volume-workspace` for utility purposes when the volume workspace is selected, which is the default on macOS. This [Docker image](./docker/campaign-volume-workspace/Dockerfile) is built by [a Python script](./docker/campaign-volume-workspace/push.py) invoked by the GitHub Action workflow described in [`docker.yml`](.github/workflows/docker.yml).

Dockerfile

@@ -1,7 +1,20 @@
+# Since we're going to provide images based on Alpine, we also want to build on
+# Alpine, rather than relying on the ./src in the surrounding environment to be
+# sane.
+#
+# Nothing fancy here: we copy in the source code and build on the Alpine Go
+# image. Refer to .dockerignore to get a sense of what we're not going to copy.
+FROM golang:1.15-alpine as builder
+
+COPY . /src
+WORKDIR /src
+RUN go build ./cmd/src
+
+# This stage should be kept in sync with Dockerfile.release.
 FROM sourcegraph/alpine:3.12@sha256:133a0a767b836cf86a011101995641cf1b5cbefb3dd212d78d7be145adde636d
 
 # needed for `src lsif upload` and `src actions exec`
 RUN apk add --no-cache git
 
-COPY src /usr/bin/
+COPY --from=builder /src/src /usr/bin/
 ENTRYPOINT ["/usr/bin/src"]

Dockerfile.release

@@ -0,0 +1,11 @@
+# This Dockerfile is used by goreleaser when publishing a release, and is not
+# suitable for testing, since it depends on a src binary being at the project
+# root _and_ that src binary being runnable on Alpine. To test this, refer to
+# the main Dockerfile, which should have an identical second stage.
+FROM sourcegraph/alpine:3.12@sha256:133a0a767b836cf86a011101995641cf1b5cbefb3dd212d78d7be145adde636d
+
+# needed for `src lsif upload` and `src actions exec`
+RUN apk add --no-cache git
+
+COPY src /usr/bin/
+ENTRYPOINT ["/usr/bin/src"]