Expand environment variable inside container on Docker command line - bash

Question

Suppose that I create a Dockerfile that just runs an echo command:
FROM alpine
ENTRYPOINT [ "echo" ]
and that I build it like this:
docker build -t my_echo .
If I run docker run --rm my_echo test it will output test as expected.
But how can I run the command to display an environment variable that is inside the container?
Example:
docker run --rm --env MYVAR=foo my_echo ???
How to access the $MYVAR variable that is in the container to display foo by replacing the ??? part of that command?
Note:
This is a simplified version of my real use case. My real use case is a WP-CLI Docker image that I built with a Dockerfile. It has the wp-cli command as the ENTRYPOINT.
I am trying to run a container based on this image to update a WordPress parameter with an environment variable. My command without Docker is wp-cli option update siteurl "http://example.com" where http://example.com would be in an environment variable.
This is the command I am trying to run (wp_cli is the name of my container):
docker run --rm --env WEBSITE_URL="http://example.com" wp_cli option update siteurl ???

Answer 1

It's possible to have the argument that immediately follows ["bash", "-c"] itself be a shell script that looks for sigils to replace. For example, consider the following script, which I'm going to call argEnvSubst:
#!/usr/bin/env bash
args=( "$#" ) # collect all arguments into a single array
for idx in "${!args[#]}"; do # iterate over the indices of that array...
arg=${args[$idx]} # ...and collect the associated values.
if [[ $arg =~ ^#ENV[.](.*)#$ ]]; then # if we have a value that matches a pattern...
varname=${BASH_REMATCH[1]} # extract the variable name from that pattern
args[$idx]=${!varname} # and replace the value with a lookup result
fi
done
exec "${args[#]}" # run our resulting array as a command.
Thus, argEnvSubst "echo" "#ENV.foobar#" will replace #ENV.foobar# with the value of the environment named foobar before it invokes echo.
While I would strongly suggest injecting this into your Dockerfile as a separate script and naming that script as your ENTRYPOINT, it's possible to do it in-line:
ENTRYPOINT [ "bash", "-c", "args=(\"$#\"); for idx in \"${!args[#]}\"; do arg=${args[$idx]}; if [[ $arg =~ ^#ENV[.](.*)#$ ]]; then varname=${BASH_REMATCH[1]}; args[$idx]=${!varname}; fi; done; \"${args[#]}\"", "_" ]
...such that you can then invoke:
docker run --rm --env WEBSITE_URL="http://example.com" \
wp_cli option update siteurl '#ENV.WEBSITE_URL#'
Note the use of bash -- this means alpine (providing only dash) isn't sufficient.