ci: derive release binary version from tag (#224)

Set the CLI build version from OBSCURA_VERSION, GitHub tag refs, or Cargo.toml fallback. Use that same build version for both --version and the startup banner, and pass the Docker tag version into Docker builds.

crates/obscura-cli/build.rs

@@ -0,0 +1,36 @@
+use std::env;
+
+fn env_value(name: &str) -> Option<String> {
+    env::var(name)
+        .ok()
+        .map(|value| value.trim().to_string())
+        .filter(|value| !value.is_empty())
+}
+
+fn normalize_version(version: String) -> String {
+    version.strip_prefix('v').unwrap_or(&version).to_string()
+}
+
+fn github_tag_version() -> Option<String> {
+    if env_value("GITHUB_REF_TYPE").as_deref() == Some("tag") {
+        return env_value("GITHUB_REF_NAME").map(normalize_version);
+    }
+
+    env_value("GITHUB_REF")
+        .and_then(|value| value.strip_prefix("refs/tags/").map(str::to_owned))
+        .map(normalize_version)
+}
+
+fn main() {
+    println!("cargo:rerun-if-env-changed=OBSCURA_VERSION");
+    println!("cargo:rerun-if-env-changed=GITHUB_REF_TYPE");
+    println!("cargo:rerun-if-env-changed=GITHUB_REF_NAME");
+    println!("cargo:rerun-if-env-changed=GITHUB_REF");
+
+    let version = env_value("OBSCURA_VERSION")
+        .map(normalize_version)
+        .or_else(github_tag_version)
+        .unwrap_or_else(|| env::var("CARGO_PKG_VERSION").expect("Cargo sets CARGO_PKG_VERSION"));
+
+    println!("cargo:rustc-env=OBSCURA_BUILD_VERSION={version}");
+}