Passing environment variables to application in a Docker container through docker-compose - docker

Question

I want to pass environment variables to the applications containerized in Docker through docker-compose. This is a VS 2017 15.3 solution Tools for Docker.
In my docker.compose.yml file I have:
app.web:
image: app.web
env_file:
- ./path.to.project/config.env
build:
context: ./path.to.project
dockerfile: Dockerfile
In config.env I have:
TEST=Compose
But when I try to read the variables using Environment.GetEnvironmentVariable("TEST"); I always get null.
If I set a non-existent file in env_file it complains when I run it, so I give for granted that is locating the file.
If I set the variable this way:
app.web:
image: app.web
environment:
- TEST=ComposeLiteral
build:
context: ./path.to.project
dockerfile: Dockerfile
I get "ComposeLiteral" when evaluating "TEST".
Which is the correct way of passing a file with environment variables to the application?

Answer 1

The problem is that the config.env file I was using was starting with the UTF8 BOM, making text editors to show me the same content, but causing docker compose to read something else.
When you add a text file through Visual Studio, the BOM is added.
I created an example project with the problem.
Then in the log is possible to see how the wrong parsing is happening:
config.env:
environment:
NUGET_FALLBACK_PACKAGES: /root/.nuget/fallbackpackages
"\uFEFFTEST": Compose
config2.env
environment:
NUGET_FALLBACK_PACKAGES: /root/.nuget/fallbackpackages
TEST: Compose
https://github.com/docker/compose/issues/5220

Answer 2

I am not able to reproduce your problem. If I start with your Dockerfile:
version: "2"
services:
app.web:
image: app.web
env_file:
- ./path.to.project/config.env
build:
context: ./path.to.project
dockerfile: Dockerfile
And in path.to.project I use the following Dockerfile:
FROM alpine
RUN apk add --update python3
COPY dumpenv.py /dumpenv.py
CMD python3 /dumpenv.py
Which runs a simple web server that does nothing but dump environment variables:
import http.server
import os
class Handler(http.server.BaseHTTPRequestHandler):
def do_GET(self):
self.send_response(200)
self.send_header('Content-type', 'text/plain')
self.end_headers()
content = []
for k, v in os.environ.items():
content.append('{:20} {}'.format(k, v))
self.wfile.write(bytes('\n'.join(content), 'utf-8'))
return
server = http.server.HTTPServer(('0.0.0.0', 8080), Handler)
server.serve_forever()
If I docker-compose up this environment and then connect to the service, I see:
PWD /
TEST ComposeLiteral
HOSTNAME 5388e6e2717a
SHLVL 1
HOME /root
PATH ...
You can find this complete example online here. If you see different behavior using this same example, could you update your question to include a complete reproducer?