Passing a JSON file as environment variable in Docker

Updated answer

You mentioned that you use the docker run command in a systemd unit file. A systemd ExecStart options is not started in a shell. Environment variable substitution is supported by name. Also see the documentation on this:

Basic environment variable substitution is supported. Use "${FOO}" as part of a word, or as a word of its own, on the command line, in which case it will be replaced by the value of the environment variable including all whitespace it contains, resulting in a single argument.

The doc also says that StartExec is not executed in a shell:

This syntax is intended to be very similar to shell syntax, but only the meta-characters and expansions described in the following paragraphs are understood. Specifically, redirection using "<", "<<", ">", and ">>", pipes using "|", running programs in the background using "&", and other elements of shell syntax are not supported. [...] Note that shell command lines are not directly supported.

However, you can use ExecStart to start a shell and then pass a command using the -c flag (you still need to quote the variable as mentioned in my original answer below):

ExecStart=/bin/bash -c "docker run -e \"TEMP_CONFIG=$(</etc/config.json)\" ..."

Original answer

Your JSON string contains spaces, and without quoting your shell will interpret everything after the first space as subsequent arguments. So TEMP_CONFIG=$(cat /etc/config.json) is essentially equivalent to:

--env TEMP_CONFIG={ "conf" : { "...

In this case, the TEMP_CONFIG environmant variable will have the value {, and docker run will assume "conf" to be the next argument (in this case, the image name).

Solution: Quote your bash variables:

--env "TEMP_CONFIG=$(cat /etc/config.json)"

Also, don't use cat when you don't have to:

--env "TEMP_CONFIG=$(</etc/config.json)"