12: Add basic CI with linters

We want PRs to have an explicit indicator for linter errors in the branch/PR. It will allow us to see the state of the branch without explicitly running linters locally if we don't want it.

In the scope of this task we need to add a basic CI pipeline with linters and status indicator on GitHub.

Steps to do:

	- add `Dockerfile` for the application (we don't want to add `ENTRYPOINT` in it for now, since we suppose that it is better to have container entrypoints inside the `docker-compose` file)
	- add `docker-compose` file
	Compose file should have following services:
	- `forum123-build` (will be used to build application)
	- `forum123-mypy` (to run `mypy`)
	- `forum123-flake8` (to run `flake8`)
	- `forum123-pylint` (to run `pylint`)
	We want them to be separated on different services to be able to run build in one job, and then run all linters in a parallel way in three different jobs (parallel execution will be implemented sometimes later, now we need only to have different services in compose file).
	- setup CI pipeline using GitHub Actions

Also we want to put this configuration in a separate folder like `envs/dev` to indicate that this is only for development purposes. And when we will need to add some production infrastructure it will go into `envs/prod`.
Seems like using different folders is more convenient than having a bunch of Dockerfile's and docker-compose files with suffixes like `.dev` and `.prod`.

We decided to put mypy stubs in `requirements-dev.txt`, because we had a troubles with installing types on CI. In our case we had to run `mypy` twice - first time, to populate `mypy`'s cache and define what types are missing, and second time to install types and check for errors. The cause of this isssue is that github always run its jobs in a new clean containers, so each new `mypy` run will not have `mypy`'s cache from the previous run. Therefore we had to run `mypy` twice for every job to have type stubs installed. More about related problems with missing type stubs on CI from other people you can read here: python/mypy#10600

Worth to mention that now we're not going to make this pipeline optimized by performance. We are planning to add caching of intermediate docker layers later in the scope of another task. For now we just need this pipeline to work and nothing more.

.github/workflows/lint.yml

@@ -0,0 +1,24 @@
+on: push
+jobs:
+  build:
+    name: build
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check out code
+        uses: actions/checkout@v3
+
+      - name: Build linters and application images
+        run: |
+          docker-compose -f envs/dev/docker-compose.yml build forum123-build
+          docker-compose -f envs/dev/docker-compose.yml build forum123-mypy
+          docker-compose -f envs/dev/docker-compose.yml build forum123-flake8
+          docker-compose -f envs/dev/docker-compose.yml build forum123-pylint
+
+      - name: Run mypy
+        run: docker-compose -f envs/dev/docker-compose.yml run forum123-mypy
+
+      - name: Run flake8
+        run: docker-compose -f envs/dev/docker-compose.yml run forum123-flake8
+
+      - name: Run pylint
+        run: docker-compose -f envs/dev/docker-compose.yml run forum123-pylint

envs/dev/Dockerfile

@@ -0,0 +1,20 @@
+FROM python:3.10-slim
+
+WORKDIR /forum123
+
+# Setup and activate python3.10 virtual environment. It will allow us to avoid this warning from pip:
+#     WARNING: Running pip as the 'root' user can result in broken permissions and conflicting behaviour with the system
+#     package manager. It is recommended to use a virtual environment instead: https://pip.pypa.io/warnings/venv
+RUN python3.10 -m venv .venv
+ENV PATH="/forum123/.venv/bin:$PATH"
+
+COPY requirements.txt requirements-dev.txt ./
+
+RUN apt-get update \
+    && pip install --upgrade pip \
+    && pip install -r requirements.txt \
+                   -r requirements-dev.txt
+
+COPY . ./
+
+ENV FLASK_APP=forum123

envs/dev/docker-compose.yml

@@ -0,0 +1,20 @@
+version: "3"
+
+services:
+
+  forum123-build: &forum123-build
+    build:
+      context: ../..  # path from the current file to the project root directory
+      dockerfile: envs/dev/Dockerfile  # path from the project root directory to the Dockerfile
+
+  forum123-mypy:
+    <<: *forum123-build
+    entrypoint: mypy
+
+  forum123-flake8:
+    <<: *forum123-build
+    entrypoint: flake8
+
+  forum123-pylint:
+    <<: *forum123-build
+    entrypoint: pylint src

requirements-dev.txt

@@ -1,4 +1,5 @@
 # Development dependencies. Please keep this list in alphabetical order.
+# Note that mypy type stubs listed in a separate section below.
 
 darglint==1.8.1  # Checks whether a docstring's description matches the actual function/method implementation
 dlint==0.12.0  # Tool for encouraging best coding practices and helping ensure Python code is secure.
@@ -28,3 +29,8 @@ flake8-type-checking==1.0.3  # Plugin for managing type-checking imports & forwa
 flake8==3.9.2
 mypy==0.991
 pylint==2.15.9
+
+
+# Mypy types. Please keep this list in alphabetical order.
+
+types-cryptography==3.3.23.2

src/routes.py

@@ -8,6 +8,6 @@
 
 @bp.route("/")
 @bp.route("/index")
-def index() -> str:  # pylint: disable=unused-variable
+def index() -> int:  # pylint: disable=unused-variable
     """Use this view function to check whether Flask is installed properly."""
     return "Hello, wordl!"