The container becomes different from the container created with run - docker

Question

When I create a container using docker run I get the expected result - a container with all the packages installed.
When I exit the container (without changing anything) and try to enter it again using docker exec, it doesn't have the packages installed.
Can Someone explain why this happens?
ubuntu#DESKTOP:~$ docker pull ciscotestautomation/pyats:latest
latest: Pulling from ciscotestautomation/pyats
Digest: sha256:7e1134ee4c7bb0d78f5aec0b71414cf1ac79026fc4757203bf29b88af24b1a03
Status: Image is up to date for ciscotestautomation/pyats:latest
docker.io/ciscotestautomation/pyats:latest
ubuntu#DESKTOP:~$ docker run --name=run_only -it ciscotestautomation/pyats:latest /bin/bash
[Entrypoint] Starting pyATS Docker Image ...
[Entrypoint] Workspace Directory: /pyats
[Entrypoint] Activating workspace
root#34db091e8cf4:/pyats# pip list
Package Version
-------------------- -----------
aiofiles 0.7.0
aiohttp 3.7.4.post0
aiohttp-swagger 1.0.15
async-lru 1.0.2
async-timeout 3.0.1
attrs 21.2.0
certifi 2021.5.30
cffi 1.14.6
chardet 4.0.0
charset-normalizer 2.0.6
cryptography 3.4.8
dill 0.3.4
distro 1.6.0
idna 3.2
idna-ssl 1.1.0
Jinja2 2.11.3
junit-xml 1.9
MarkupSafe 1.1.1
multidict 5.1.0
pathspec 0.9.0
pip 21.2.4
psutil 5.8.0
pyats 21.9
pyats.aereport 21.9
pyats.aetest 21.9
pyats.async 21.9
pyats.connections 21.9
pyats.datastructures 21.9
pyats.easypy 21.9
pyats.kleenex 21.9
pyats.log 21.9
pyats.reporter 21.9
pyats.results 21.9
pyats.tcl 21.9
pyats.topology 21.9
pyats.utils 21.9
pycparser 2.20
python-engineio 3.14.2
python-socketio 4.6.1
PyYAML 5.4.1
requests 2.26.0
setuptools 58.1.0
six 1.16.0
typing-extensions 3.10.0.2
unicon 21.9
unicon.plugins 21.9
urllib3 1.26.7
wheel 0.37.0
yamllint 1.26.3
yarl 1.6.3
root#34db091e8cf4:/pyats# exit
exit
ubuntu#DESKTOP:~$ docker ps -a
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
34db091e8cf4 ciscotestautomation/pyats:latest "/bin/tini -- /entry…" 14 seconds ago Exited (0) 4 seconds ago run_only
ubuntu#DESKTOP:~$ docker start run_only
run_only
ubuntu#DESKTOP:~$ docker ps -a
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
34db091e8cf4 ciscotestautomation/pyats:latest "/bin/tini -- /entry…" 36 seconds ago Up 2 seconds run_only
ubuntu#DESKTOP:~$ docker exec -it run_only "/bin/bash"
root#34db091e8cf4:/pyats# pip list
Package Version
---------- -------
pip 21.1.3
setuptools 57.0.0
wheel 0.36.2
WARNING: You are using pip version 21.1.3; however, version 21.2.4 is available.
You should consider upgrading via the '/usr/local/bin/python -m pip install --upgrade pip' command.
root#34db091e8cf4:/pyats#

Answer 1

When you docker run the image, you'll note the following output:
[Entrypoint] Starting pyATS Docker Image ...
[Entrypoint] Workspace Directory: /pyats
[Entrypoint] Activating workspace
There is some sort of initialization script that is activating the workspace (probably a Python virtual environment).
When you docker exec into the container, you're just starting a shell, without any initialization. You need to run whatever initialization steps are performed when the container starts.
If you docker inspect the image, you can see that the ENTRYPOINT is set to /bin/tini -- /entrypoint.sh, and if you take a look at /entrypoint.sh, you find this, which includes:
echo "[Entrypoint] Activating workspace"
source ${WORKSPACE}/bin/activate
(Where $WORKSPACE is an environment variable that will already be set in your shell.)