Dockerfile set runtime ENV dinamically by sourcing a script

Question

Basically, I need to keep the functionality of an ubuntu:18.04 image but with some environment variables set every time I execute a docker run or a docker exec this variables are dynamic, so I can't use the keyword ENV in the Dockerfile, I will need to use a script that should be sourced, for simplicity the file I will be using for this post is:

$ cat setenv.sh 
#!/usr/bin/env bash

# Set some dynamic variables
export TEST="Hello World"

I have tried different approaches without success, here is my research:

Using an entrypoint

The files I used for this example:

$ cat entrypoint.sh 
#!/usr/bin/env bash

echo "Setting environment"
. /setenv.sh
exec $@

$ cat Dockerfile 
FROM ubuntu:18.04

COPY setenv.sh /
COPY entrypoint.sh /

ENTRYPOINT [ "/entrypoint.sh" ]

I built this Dockerfile the following command: docker build -f Dockerfile -t test_img .

This works fine except by two problems:

1. exec does not support double ampersand && nor pipes | nor escaping chars \

As I previously stated, I require my container to have the same functionality as the ubuntu image, for example, in ubuntu I can totally execute the following container:

$ docker run --rm ubuntu:18.04 bash -c "echo \"Hello World\" && ls | head -n1 "
Hello World
bin

But if I use the image I created:

$ docker run --rm test_img bash -c "echo \"Hello World\" && ls | head -n1"
Setting environment

It truncates the command every time it finds a quote (doesn't honor the escape character) a double ampersand or a pipe, here is an example of the commands in different order:

$ docker run --rm ubuntu:18.04 bash -c "ls | head -n1 && echo \"Hello World\""
bin
Hello World

$ docker run --rm test_img bash -c "ls | head -n1 && echo \"Hello World\""
Setting environment
bin
boot
dev
entrypoint.sh
etc
home
lib
lib64
media
mnt
opt
proc
root
run
sbin
setenv.sh
srv
sys
tmp
usr
var

In this case, the command truncates when finding the pipe |.

2. Entrypoint is only called for the parent shell.

If I run a ephemeral container I can see that my env variable is there:

$ docker run --rm test_img env | grep TEST
TEST=Hello World

But if I want a keep-alive container, the env var is not set:

$ docker create -ti --name=test test_img bash
e0e5278c46bdcf33195661fac5911326b701586e9a9c638f71a6e08021ee2f57
$ docker start test
test
$ docker exec test env | grep TEST

What is happening here is that the shell I create when running docker create is calling the entrypoint, but the shell I create when running docker exec is a different one.

If you login to the container you can see shells are different:

$ docker exec -ti test bash
root@e0e5278c46bd:/# ps -fe
UID          PID    PPID  C STIME TTY          TIME CMD
root           1       0  0 15:21 pts/0    00:00:00 bash
root          15       0  0 15:29 pts/1    00:00:00 bash
root          29      15  0 15:29 pts/1    00:00:00 ps -fe
root@e0e5278c46bd:/# env | grep TEST

If instead of having an entrypoint script to set the environment variable TEST I had used the keyword ENV in my Dockerfile: ENV TEST "Hello World" this would set the variable in every shell created by the commands docker run and docker exec. Here is the example:

$ cat Dockerfile 
FROM ubuntu:18.04

ENV TEST "Hello World"

$ docker build -f Dockerfile -t test_img .
Sending build context to Docker daemon  4.096kB
Step 1/2 : FROM ubuntu:18.04
 ---> 6526a1858e5d
Step 2/2 : ENV TEST "Hello World"
 ---> Using cache
 ---> eebe9952bb76
Successfully built eebe9952bb76
Successfully tagged test_img:latest

$ docker create -ti --name=test test_img bash
c1e508dae0f398a40c4c5534cf2811cdfe284a4f6601198f0ca97fdea100c376
$ docker start test
test
$ docker exec test env | grep TEST
TEST=Hello World
$ docker exec -ti test bash
root@c1e508dae0f3:/# env | grep TEST
TEST=Hello World

Sourcing in bashrc

I modify the Dockerfile to look like this, and built the image with the same build command:

$ cat Dockerfile 
FROM ubuntu:18.04

COPY setenv.sh /
RUN echo ". /setenv.sh" >> /etc/bash.bashrc

The problem with this approach is the shell used to execute docker run, the bashrc file is not called, only on interactive bash shells, here is the output:

$ docker run --rm test_img echo $SHELL
/bin/bash
$ docker run --rm test_img env | grep TEST
$ docker run --rm test_img bash -c "env" | grep TEST
$ docker run --rm -ti test_img bash
root@1187568e1bec:/# env | grep TEST
TEST=Hello World

First I tried to add the setenv.sh to /etc/profile.d directory, but the problem with this is that /etc/profile is called only for login shells, and I will need to change the commands to explicitly use a login shell, in other words, instead of docker run test_img env I would need it to be docker run test_img bash -lc "env" (The -l is for login).

Create Dockerfile dinamically

This is the best solution so far, but is not the cleaner, I have to have a Dockerfile.pre file to create a container and save the generated variables to a file, then use this file to create a final Dockerfile and write all those ENV lines into the Dockerfile.

Combining two approaches

By using an entrypoint and sourcing in bashrc file I was able to get the variables set in all cases, the problem is the exec $@ command that doesn't support full bash scripts. Is any way to modify my entrypoint script? or is there other approach for this problem?

Answer 1

you can create an enviroment file and just pass it to your container with the --env-file flag. This will make all the variables in the file available in the container.

ubuntu@vps-f116ed9f:~$ cat my_env_file
TEST=Hello World

ubuntu@vps-f116ed9f:~$ docker container run -it --rm --env-file my_env_file ubuntu bash -c "echo \$TEST"
Hello World

ubuntu@vps-f116ed9f:~$ docker container run -it --rm --env-file my_env_file ubuntu bash -c "echo \$TEST | wc -c"
12

here you can see i have used the latest ubuntu image, i pass my_env_file to it and then using the bash shell i print the value of this variable (Note i have to escape the $ other wise the shell will interpolate this before passing it to docker, this could be avoided by using single qoutes as the shell wont interpolate variables in single qoutes.)

I also dont see any issues using pipe or &&

ubuntu@vps-f116ed9f:~$ docker container run -it --rm --env-file my_env_file ubuntu bash -c 'ls | head -n1 && echo "$TEST"'
bin
Hello World

This also will persist in detached containers

ubuntu@vps-f116ed9f:~$ docker container run -itd --rm --name=c1 --env-file my_env_file ubuntu bash
3d7705f2f91f3f30c45e855778bd80f08a35616bbe822545c20d5a8886139693

ubuntu@vps-f116ed9f:~$ docker container exec c1 sh -c "ls | head -1 && echo \$TEST"
bin
Hello World