Dockerfile doesn't source .bashrc even in a single subshell - bash

Question

I'm trying to source .bashrc but no luck
USER user
SHELL ["/bin/bash", "-c"]
RUN echo "export TEST_VAR=test" >> /home/user/.bashrc && tail /home/user/.bashrc && source /home/user/.bashrc && echo "1 \"${TEST_VAR} 2\" var" && exit 1
I expect that this RUN command print 1 "test" 2 but what i get is that
Step 13/40 : RUN echo "export TEST_VAR=test" >> /home/user/.bashrc && tail /home/user/.bashrc && source /home/user/.bashrc && echo "1 \"${TEST_VAR}\" 2" && exit 1
---> Running in b870d36e9dd0
# this, if it's already enabled in /etc/bash.bashrc and /etc/profile
# sources /etc/bash.bashrc).
if ! shopt -oq posix; then
if [ -f /usr/share/bash-completion/bash_completion ]; then
. /usr/share/bash-completion/bash_completion
elif [ -f /etc/bash_completion ]; then
. /etc/bash_completion
fi
fi
export TEST_VAR=test
1 "" 2
What's wrong with handling shells in docker? I just wanted to source ~/.bashrc once and use all exposed variables in subsequent command below source call but it doesn't even work in a single subshell joined with &&

Answer 1

Usually ~/.bashrc contains something similar to:
# If not running interactively, don't do anything
case $- in
*i*) ;;
*) return;;
esac
That is very normal - .bashrc is meant to be used in interactive sessions only. Because RUN is non-interactive, it just exits.
Aaaanyway, I would recommend, if you want to only add environment variables, output them to /etc/profile.d and . /etc/profile.

Answer 2

Most paths in Docker don't read shell dotfiles at all. You need to use other approaches to provide configuration to your application; for example, Dockerfile ENV to set environment variables or an entrypoint wrapper script if you need things to be set up dynamically before starting the container.
Let's look specifically at a reduced form of your example:
SHELL ["/bin/bash", "-c"]
RUN echo "export TEST_VAR=test" >> $HOME/.bashrc
RUN echo "$TEST_VAR"
Bash Startup Files in the GNU Bash manual lists out which dotfiles are read in which case. For the last line Docker combines the SHELL and RUN lines to run the equivalent of
/bin/bash -c 'echo "$TEST_VAR"'
but the bash instance is neither an interactive nor a login shell, so the only dotfile that's automatically read is one named in a $BASH_ENV environment variable. (POSIX sh doesn't specify anything about any shell dotfiles at all.)
This further applies to the image's default CMD, which also will get run with sh -c (or the alternate SHELL) and it won't read dotfiles. If the CMD (or ENTRYPOINT or RUN) uses JSON-array syntax, it won't invoke a shell at all, and again won't read dotfiles.
The only case where shell dotfiles will be read is if the main container command is an interactive shell, and this won't typically be the common case.
docker run --rm -it yourimage /bin/bash # reads .bashrc
docker run --rm -it yourimage /bin/bash --login # also reads .profile, .bash_login
This means you should almost never try to edit the .bashrc, /etc/profile, or any similar files. If you need to set environment variables as in the example, use Dockerfile ENV instead.
ENV TEST_VAR=test
RUN echo "$TEST_VAR"