diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index a4a876a..f12d583 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -145,8 +145,7 @@ subplot:
   script:
     # make sure a virtual card is available, so that the subplot tests are
     # generated
-    - touch tools/virtual-card-available
-    - cargo test
+    - CARD_BASED_TESTS=true cargo test
   cache:
     # inherit all general cache settings
     <<: *general_cache_config
diff --git a/tools/build.rs b/tools/build.rs
index 608f687..5185c03 100644
--- a/tools/build.rs
+++ b/tools/build.rs
@@ -9,19 +9,23 @@ fn main() {
     // card is available. This is a kludge until Subplot can do
     // conditional scenarios
     // (https://gitlab.com/subplot/subplot/-/issues/20).
-    let flagfile = Path::new("virtual-card-available");
-    if flagfile.exists() {
-        subplot_build::codegen("subplot/opgpcard.subplot")
-            .expect("failed to generate code with Subplot");
-    } else {
-        // If we're not generating code from the subplot, we should at
-        // least create an empty file so that the tests/opgpcard.rs
-        // file can include it. Otherwise the build will fail.
-        let out_dir = std::env::var("OUT_DIR").unwrap();
-        let include = Path::new(&out_dir).join("opgpcard.rs");
-        eprintln!("build.rs: include={}", include.display());
-        if !include.exists() {
-            File::create(include).unwrap();
+    match option_env!("CARD_BASED_TESTS") {
+        Some(_) => {
+            subplot_build::codegen("subplot/opgpcard.subplot")
+                .expect("failed to generate code with Subplot");
+            println!("cargo:warning=generated subplot tests");
+        }
+        None => {
+            // If we're not generating code from the subplot, we should at
+            // least create an empty file so that the tests/opgpcard.rs
+            // file can include it. Otherwise the build will fail.
+            println!("cargo:warning=did not generate subplot tests");
+            let out_dir = std::env::var("OUT_DIR").unwrap();
+            let include = Path::new(&out_dir).join("opgpcard.rs");
+            eprintln!("build.rs: include={}", include.display());
+            if !include.exists() {
+                File::create(include).unwrap();
+            }
         }
     }
 }
diff --git a/tools/cargo-test-in-docker b/tools/cargo-test-in-docker
index 945294f..9332cd8 100755
--- a/tools/cargo-test-in-docker
+++ b/tools/cargo-test-in-docker
@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 #
 # Run this to run opgpcard (tools directory) test suite inside a
 # Docker container with a virtual smartcard running. The test suite
@@ -14,10 +14,12 @@ docker run --rm -it \
 	-v root:/root \
 	-v cargo:/cargo \
 	-v $(pwd):/src \
+	-e CARD_BASED_TESTS=true \
 	registry.gitlab.com/openpgp-card/virtual-cards/smartpgp-builddeps sh -c '
 sed -i "s/timeout=20/timeout=60/" /home/jcardsim/run-card.sh &&
 /etc/init.d/pcscd start &&
 su - -c "sh /home/jcardsim/run-card.sh >/dev/null" jcardsim &&
 cd /src/tools &&
-if ! [ -e virtual-card-available ]; then rm -f tests/opgpcard.rs; fi &&
+CARGO_TARGET_DIR=/cargo/ cargo update &&
+CARGO_TARGET_DIR=/cargo/ cargo build -vv &&
 CARGO_TARGET_DIR=/cargo/ cargo test'
diff --git a/tools/subplot/opgpcard.md b/tools/subplot/opgpcard.md
index de1693f..d0729d8 100644
--- a/tools/subplot/opgpcard.md
+++ b/tools/subplot/opgpcard.md
@@ -17,9 +17,9 @@ The verification scenarios in this document assume the availability of
 a virtual smart card. Specifically one described in
 <https://gitlab.com/openpgp-card/virtual-cards>. The
 `openpgp-card/tools` crate is set up to generate tests only if the
-file `tools/virtual-card-available` exists, and the `openpgp-card`
-repository `.gitlab-ci.yml` file is set up to create that file when
-the repository is tested in GitLab CI.
+environment variable `CARD_BASED_TESTS` is set (to any value),
+and the `openpgp-card` repository `.gitlab-ci.yml` file is set up to
+set that environment variable when the repository is tested in GitLab CI.
 
 This means that if you run `cargo test`, no test code is normally
 generated from this document. To run the tests locally, outside of
diff --git a/tools/subplot/test-in-docker.sh b/tools/subplot/test-in-docker.sh
index 13bf2e1..c24ae9a 100755
--- a/tools/subplot/test-in-docker.sh
+++ b/tools/subplot/test-in-docker.sh
@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 #
 # Run "cargo test" inside a Docker container with virtual cards
 # installed and running. This will allow at least rudimentary testing