REP-6296 Add a GitHub release process (#121)

This:
- adds a GitHub Action that runs whenever a `v`-prefixed semver tag is pushed
- adds documentation for the release process
- reworks the `build.sh` script to prefer the release version rather than the git revision

Author: FGasper

.github/workflows/release.yml

@@ -0,0 +1,42 @@
+name: Release
+
+on:
+  push:
+    tags:
+      - 'v*.*.*'
+
+env:
+  TARGETS: |
+    linux/amd64
+    linux/arm64
+    darwin/arm64
+
+permissions:
+  contents: write
+
+jobs:
+  build-and-release:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Set up Go
+        uses: actions/setup-go@v4
+
+      - name: Build
+        run: |
+          for target in $TARGETS; do
+            export GOOS=${target%/*}
+            export GOARCH=${target#*/}
+            ./build.sh
+            mv migration_verifier migration_verifier_${GOOS}_${GOARCH}
+          done
+
+      - name: Create Release
+        uses: softprops/action-gh-release@v2
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        with:
+          files: migration_verifier_*

RELEASE.md

@@ -0,0 +1,16 @@
+# Releasing Migration Verifier
+
+To release Migration Verifier, just create a lightweight git tag, thus:
+```
+git tag v0.0.2
+```
+… and push it to upstream.
+
+Versions **MUST** start with a `v` and follow
+[semantic versioning](https://semver.org/).
+
+An automated release process will build binaries and make them available
+for download. Check GitHub Actions for progress.
+
+Note that this process **DOES NOT** create release notes. Users should peruse
+the repository’s commit history for changes.

build.sh

@@ -1,8 +1,13 @@
 #!/bin/sh
 
-commit=$(git show --no-patch --format='%H')
+revision=$(git describe --tags --exact-match 2>/dev/null || echo "DEVELOPMENT:$(git describe --tags)")
 buildTime=$(date -u)
 
-printf 'Building migration-verifier …\n\tcommit: %s\n\tbuildTime: %s\n' "$commit" "$buildTime"
+goos=$(go env GOOS)
+goarch=$(go env GOARCH)
 
-go build -ldflags="-X 'main.Revision=$commit' -X 'main.BuildTime=$buildTime'" main/migration_verifier.go
+printf 'Building migration-verifier for %s/%s …\n' "$goos" "$goarch"
+printf '\tRevision: %s\n' "$revision"
+printf '\tBuild Time: %s\n' "$buildTime"
+
+go build -ldflags="-X 'main.Revision=$revision' -X 'main.BuildTime=$buildTime'" main/migration_verifier.go