Provide more metadata with images

This assigns the OCI spec labels to images pushed to GHCR, and enables
image users to see current tool version instead of just v0+HEAD.

.github/workflows/docker.yml

@@ -43,3 +43,13 @@ jobs:
           tags: |
             ghcr.io/${{ github.repository }}:latest
             ghcr.io/${{ github.repository }}:${{ env.tag_name }}
+          build-args: |
+            gmnhg_version=${{ env.tag_name }}
+          # TODO: set org.opencontainers.image.revision when this gets
+          # fixed: https://github.com/actions/checkout/issues/209
+          labels: |
+            org.opencontainers.image.url="${{ github.server_url }}/${{ github.repository }}"
+            org.opencontainers.image.source=${{ github.repositoryUrl }}
+            org.opencontainers.image.version=${{ env.tag_name }}
+            org.opencontainers.image.licenses=GPL-3.0
+            org.opencontainers.image.title=gmnhg

Dockerfile

@@ -3,16 +3,19 @@ FROM docker.io/library/golang:1.18-alpine AS builder
 WORKDIR /src
 
 ENV CGO_ENABLED=0
-ENV GO111MODULE=on
-ENV GOFLAGS="-trimpath -ldflags=-linkmode=external"
 
 RUN apk add --no-cache git gcc musl-dev
 
 COPY . /src/
 
-RUN \
-    go build -v -o /tmp/gmnhg ./cmd/gmnhg && \
-    go build -v -o /tmp/md2gmn ./cmd/md2gmn
+ARG gmnhg_version=v0+HEAD
+RUN go build \
+    -v \
+    -trimpath \
+    -ldflags="-linkmode=external -X main.version=${gmnhg_version}" \
+    -o /tmp/ \
+    ./cmd/gmnhg \
+    ./cmd/md2gmn
 
 FROM docker.io/library/alpine:3.15 AS runner