Running "/usr/local/bin/gunicorn" in a docker build says " stat /usr/local/bin/gunicorn: no such file or directory"

Question

From the toplevel maps directory, I'm able to install the gunicorn extension ...

(venv) localhost:maps davea$ pip3 install gunicorn
Collecting gunicorn
  Downloading gunicorn-20.0.4-py2.py3-none-any.whl (77 kB)
     |████████████████████████████████| 77 kB 1.2 MB/s 
Requirement already satisfied: setuptools>=3.0 in ./web/venv/lib/python3.7/site-packages (from gunicorn) (45.1.0)
Installing collected packages: gunicorn
Successfully installed gunicorn-20.0.4

Below is my docker-compose.yml file

version: '3'

services:
  web:
    restart: always
    build: ./web
    ports:           # to access the container from outside
      - "8000:8000"
    environment:
      DEBUG: 'true'
    command: /usr/local/bin/gunicorn maps.wsgi:application -w 2 -b :8000

  apache:
    restart: always
    build: ./apache/
    ports:
      - "80:80"
    #volumes:
    #  - web-static:/www/static
    links:
      - web:web

  mysql:
    restart: always
    image: mysql:5.7
    environment:
      MYSQL_DATABASE: 'maps_data'
      # So you don't have to use root, but you can if you like
      MYSQL_USER: 'chicommons'
      # You can use whatever password you like
      MYSQL_PASSWORD: 'password'
      # Password for root access
      MYSQL_ROOT_PASSWORD: 'password'
    ports:
      - "3406:3406"
    volumes:
      - my-db:/var/lib/mysql

volumes:
  my-db:

And then I have web/Dockerfile as follows ...

FROM python:3.7-slim

RUN apt-get update && apt-get install

RUN apt-get install -y libmariadb-dev-compat libmariadb-dev
RUN apt-get update \
    && apt-get install -y --no-install-recommends gcc \
    && rm -rf /var/lib/apt/lists/*

RUN python -m pip install --upgrade pip
RUN mkdir -p /app/

WORKDIR /app/

RUN pip3 freeze > requirements.txt
COPY requirements.txt requirements.txt
RUN python -m pip install -r requirements.txt

COPY . /app/

However, when I build/start my docker instance, I'm told it can't find my "gunicorn" command ...

(venv) localhost:maps davea$ docker-compose up
Starting maps_web_1 ... 
Starting maps_web_1 ... error

ERROR: for maps_web_1  Cannot start service web: OCI runtime create failed: container_linux.go:346: starting container process caused "exec: \"/usr/local/bin/gunicorn\": stat /usr/local/bin/gunicorn: no such file or directory": unknown

ERROR: for web  Cannot start service web: OCI runtime create failed: container_linux.go:346: starting container process caused "exec: \"/usr/local/bin/gunicorn\": stat /usr/local/bin/gunicorn: no such file or directory": unknown
ERROR: Encountered errors while bringing up the project.

Answer 1

Your Docker container is a totally isolated environment. Nothing you install on the host is visible inside the container; nothing that happens inside the container is accessible on the host. There's ways to bridge this boundary (with docker run -v bind mounts) but that's not possible during the docker build phase.

In this example your local source tree has a requirements.txt file that lists out the packages that need to be installed when the container is created. (The RUN pip freeze line has no effect; the COPY on the line after it copies it from your local source tree.) It's enough to add the dependency to the requirements.txt file

gunicorn

In your development environment, you can re-run pip install -r requirements.txt to update the packages installed in your virtual environment. When you re-run docker build, having this line in the requirements.txt file will cause it to be installed when the package is built.

You can clean up the Dockerfile a little bit. The resulting Dockerfile would be a pretty typical one for Python packages with C dependencies:

# Start from a totally clean environment with Python installed,
# but no non-system libraries and nothing from your host system.
FROM python:3.7-slim

# Install C dependencies.
# It's important to do apt-get update and install in the
# same command.  It's more efficient to only do it once.
RUN apt-get update \
 && apt-get install -y --no-install-recommends \
      gcc \
      libmariadb-dev \
      libmariadb-dev-compat

# Update pip
RUN python -m pip install --upgrade pip

# Create the application directory and point there
# (WORKDIR will implicitly create it)
WORKDIR /app/

# Install all of the Python dependencies.  These are
# listed, one to a line, in the requirements.txt file,
# possibly with version constraints.  Having this as
# a separate block allows Docker to not repeat it if
# only your application code changes.
COPY requirements.txt .
RUN python -m pip install -r requirements.txt

# Copy in the rest of the application.
COPY . .

# Specify what port your application uses, and the
# default command to use when launching the container.
EXPOSE 8000
CMD /usr/local/bin/gunicorn maps.wsgi:application -w 2 -b :8000