added readme

README.md

@@ -4,6 +4,53 @@ Need both manual weebohook and app.
 
 Installation id needs to be configured manually.
 
+## Usage
+
+You want to have some predefined docker container with maximum dependencies cached.
+
+
+Exmaple:
+
+```dockerfile
+FROM fpco/stack-build:lts-14.27
+
+RUN stack update
+RUN stack build tasty tasty-hspec aeson containers bytestring
+
+WORKDIR /
+```
+
+You need to build that container and git it a name:
+
+```bash
+docker build -t fast-lts-14.27 - < FastDockerFile
+```
+
+After that you can use it in your `tasks.yaml`:
+
+```yaml
+test-task:
+  prebuild: |
+    cp -f ../../test-task/package.yaml .
+    cp -f ../../test-task/stack.yaml .
+    cp -f -R ../../test-task/test .
+  dockerfile: |
+    FROM fast-lts-14.27
+    COPY . src
+    WORKDIR /src
+    ENTRYPOINT stack test --fast
+  timeoutMinutes: 10
+```
+
+The docker container is expected to write json to stdout, conating test results:
+
+```json
+{
+  "test1": true,
+  "test2": false
+}
+```
+
 For macos docker API:
 
 ```bash