fix(email): address postguard#197 deliverability + show disclosed signer name

Dockerfile

@@ -11,6 +11,7 @@ WORKDIR /app
 FROM chef AS planner
 COPY Cargo.toml ./Cargo.toml
 COPY Cargo.lock ./Cargo.lock
+COPY build.rs   ./build.rs
 COPY src        ./src
 COPY templates  ./templates
 RUN cargo chef prepare --recipe-path recipe.json
@@ -25,6 +26,7 @@ RUN cargo chef cook --profile ${CARGO_PROFILE} --recipe-path recipe.json
 # Copy sources and build the application binary
 COPY Cargo.toml ./Cargo.toml
 COPY Cargo.lock ./Cargo.lock
+COPY build.rs   ./build.rs
 COPY src        ./src
 COPY templates  ./templates
 RUN cargo build --profile ${CARGO_PROFILE} --bin cryptify

build.rs

@@ -0,0 +1,31 @@
+fn main() {
+    println!("cargo:rerun-if-changed=Cargo.lock");
+
+    let lock = std::fs::read_to_string("Cargo.lock").expect("Cargo.lock not readable");
+    let version = lock
+        .split("[[package]]")
+        .find_map(|block| {
+            let mut name = None;
+            let mut ver = None;
+            for line in block.lines() {
+                if let Some(rest) = line.strip_prefix("name = \"") {
+                    name = rest.strip_suffix('"');
+                }
+                if let Some(rest) = line.strip_prefix("version = \"") {
+                    ver = rest.strip_suffix('"');
+                }
+            }
+            if name == Some("pg-core") {
+                ver
+            } else {
+                None
+            }
+        })
+        .expect(
+            "pg-core entry not found in Cargo.lock — PG_CORE_VERSION feeds the \
+             X-PostGuard mail header that the Outlook add-in's OnMessageRead \
+             launch event filters on (see src/email.rs::XPostGuard).",
+        );
+
+    println!("cargo:rustc-env=PG_CORE_VERSION={}", version);
+}