Use multistage docker build (#32)

* Use multistage docker build

* Pin docker base to the manifest SHA, to ensure multi-platform images get
built correctly.

See this SO answer: https://stackoverflow.com/questions/74761737/how-to-use-a-multiarch-base-docker-image-when-specifying-sha

The manifest information for alpine base image:

```
Name:      docker.io/library/golang:1.20.3-alpine
MediaType: application/vnd.docker.distribution.manifest.list.v2+json
Digest:    sha256:08e9c086194875334d606765bd60aa064abd3c215abfbcf5737619110d48d114

Manifests:
  Name:      docker.io/library/golang:1.20.3-alpine@sha256:48c87cd759e3342fcbc4241533337141e7d8457ec33ab9660abe0a4346c30b60
  MediaType: application/vnd.docker.distribution.manifest.v2+json
  Platform:  linux/amd64

  Name:      docker.io/library/golang:1.20.3-alpine@sha256:0f9d434ef44b3d2284ede2cdda59917584db48285a76d582a3b46e59300304ba
  MediaType: application/vnd.docker.distribution.manifest.v2+json
  Platform:  linux/arm/v6

  ...

```

* Add clarification on how to update the base image digest

Dockerfile

@@ -1,3 +1,23 @@
-FROM golang:1.21.5@sha256:135ba6c2ae2a9a022f53f3eee24ef213bc62e876811cd9582b91045168206873
+# When updating the version of the base container, please use the 
+# SHA256 of the distribution manifest, not individual images.
 
-COPY scip-go /usr/bin/
+# This ensures that when pulling the container, Docker will detect 
+# the platform and pull the correct image (if it exists)
+
+# For example, to find out the hash of the manfiest, run:
+
+# $ docker buildx imagetools inspect golang:1.21.5-alpine
+# Name:      docker.io/library/golang:1.21.5-alpine
+# MediaType: application/vnd.docker.distribution.manifest.list.v2+json
+# Digest:    sha256:4db4aac30880b978cae5445dd4a706215249ad4f43d28bd7cdf7906e9be8dd6b
+# And use this digest in FROM
+FROM golang:1.21.5@sha256:4db4aac30880b978cae5445dd4a706215249ad4f43d28bd7cdf7906e9be8dd6b
+
+COPY . /sources
+WORKDIR /sources
+RUN go build -o scip-go ./cmd/scip-go
+
+FROM alpine:latest
+
+COPY --from=builder /sources/scip-go /usr/bin/
+CMD ["scip-go"]

README.md

@@ -75,6 +75,14 @@ scip-go
 src code-intel upload -github-token='${{ secrets.GITHUB_TOKEN }}' -no-progress
 ```
 
+## Docker
+
+To build a self-contained Docker container with the indexer, use 
+the following command:
+
+```bash
+docker build -t scip-go:latest .
+```
 
 # Contributing