Require environment variables to be given to image when run using `-e`

In detach mode it not possible to print message that env is required, in your word when running with -d, but you can try a workaround:

Dockerfile

FROM alpine
COPY entrypoint.sh /usr/bin/
RUN chmod +x /usr/bin/entrypoint.sh
ENTRYPOINT ["entrypoint.sh"]

entrypoint.sh

#!/bin/sh
echo "starting container $hostname"
if [ -z "$REQUIRED_ENV" ]; then
  echo "Container failed to start, pls pass -e REQUIRED_ENV=sometest"
  exit 1
fi
echo "starting container with $REQUIRED_ENV"
#your long-running command from CMD
exec "$@"

So when you run with

docker run -it --name envtest  --rm env-test-image 

it will exit with the message

starting container 
Container failed to start, pls pass -e REQUIRED_ENV=sometest

The workaround with detach mode

docker run -it --name envtest  -d --rm env-test-image && docker logs envtest

No: there is currently no way to make Docker aware of "dependencies" in the form of environment variables. This answer is for those who actually came here just looking for a canonical way to exit an entrypoint script in case of missing values.

A POSIX shell takes the option -u (error when expanding unset variables) and -e (exit the shell when any statement returns an error/non-zero exit code). We also have : to evaluate-but-not-invoke an expression. Putting these together, we should be able to make an entrypoint.sh that works irrespective of sh-implementation like this:

#!/bin/sh -e -u

( : $USERNAME )
( : $PASSWORD ) 

exec "$@"

which will exit the script with an error akin to PASSWORD: parameter not set.

Another way of doing the same thing, but in context of the original example with a custom error message, could make use of the "Display Error if Null or Unset" operator (${parm:?error}) and look something like so:

#!/bin/sh
echo "starting container $hostname"
( : ${REQUIRED_ENV?"pls pass -e REQUIRED_ENV=sometest"} ) || exit 1
echo "starting container with $REQUIRED_ENV"
#your long-running command from CMD
exec "$@"