sed not working in dockerfile but in container bash it does - bash

Question

I'm creating a Docker image where I use sed to modify two parameters, but when i create the images and check the file I wanted to modify it remanis the same. If i run the very sed command interactively, i t works. Why? Could sombebody help me make ma image work without having to modify every container.
Dockerfile
FROM python:slim-buster
WORKDIR /home/scr_dca
COPY . .
ENV FLASK_APP Screenly.py
RUN apt-get update && \
apt install curl gnupg -y && \
curl https://packages.microsoft.com/keys/microsoft.asc | apt-key add - && \
curl https://packages.microsoft.com/config/debian/10/prod.list > /etc/apt/sources.list.d/mssql-release.list && \
apt-get update && ACCEPT_EULA=Y apt-get install msodbcsql17 unixodbc-dev -y && \
apt-get install libgssapi-krb5-2 g++ gcc && \
pip3 install -r requirements.txt --trusted-host pypi.python.org
RUN sed -i "s/\(MinProtocol *= *\).*/\1TLSv1.0 /" "/etc/ssl/openssl.cnf" && \
sed -i "s/\(CipherString *= *\).*/\1DEFAULT#SECLEVEL=1 /" "/etc/ssl/openssl.cnf"
CMD ["gunicorn", "-b", ":8000", "scr_dca:app"]
I'm doing
docker run --name mycontainer -d -p 5050:8000 src_dca_v1.0
docker container exec -it mycontainer bash
:/home/myapp# cat /etc/ssl/openssl.cnf
I checked and sed didnt work during the image creation so I ran the following commands:
sed -i "s/\(MinProtocol *= *\).*/\1TLSv1.0 /" "/etc/ssl/openssl.cnf"
sed -i "s/\(CipherString *= *\).*/\1DEFAULT#SECLEVEL=1 /" "/etc/ssl/openssl.cnf"
original part of the file I want to modify:
[system_default_sect]
MinProtocol = TLSv1.2
CipherString = #SECLEVEL=1
sed expected result
[system_default_sect]
MinProtocol = TLSv1.0
CipherString = DEFAULT#SECLEVEL=1

Answer 1

I suspect there is something you are not seeing or that you did not explain/describe in your question. As is, I cannot reproduce your problem.
My MCVE, inspired by your current question to test:
FROM python:slim-buster
RUN cp /etc/ssl/openssl.cnf /etc/ssl/openssl.cnf.ORI && \
sed -i "s/\(MinProtocol *= *\).*/\1TLSv1.0 /" "/etc/ssl/openssl.cnf" && \
sed -i "s/\(CipherString *= *\).*/\1DEFAULT#SECLEVEL=1 /" "/etc/ssl/openssl.cnf" && \
(diff -u /etc/ssl/openssl.cnf.ORI /etc/ssl/openssl.cnf || exit 0)
Note: I ignored diff exit status and force it to 0, as it will exit with status 1 when there is a difference between the files which would fail the build.
And the result:
$ docker build --no-cache -t test:test .
Sending build context to Docker daemon 4.096kB
Step 1/2 : FROM python:slim-buster
---> 3d8f801fc3db
Step 2/2 : RUN cp /etc/ssl/openssl.cnf /etc/ssl/openssl.cnf.ORI && sed -i "s/\(MinProtocol *= *\).*/\1TLSv1.0 /" "/etc/ssl/openssl.cnf" && sed -i "s/\(CipherString *= *\).*/\1DEFAULT#SECLEVEL=1 /" "/etc/ssl/openssl.cnf" && (diff -u /etc/ssl/openssl.cnf.ORI /etc/ssl/openssl.cnf || exit 0)
---> Running in 523ddc0f4025
--- /etc/ssl/openssl.cnf.ORI 2020-01-09 16:21:44.667348574 +0000
+++ /etc/ssl/openssl.cnf 2020-01-09 16:21:44.675348574 +0000
## -358,5 +358,5 ##
system_default = system_default_sect
[system_default_sect]
-MinProtocol = TLSv1.2
-CipherString = DEFAULT#SECLEVEL=2
+MinProtocol = TLSv1.0
+CipherString = DEFAULT#SECLEVEL=1
Removing intermediate container 523ddc0f4025
---> 88c28529ceb5
Successfully built 88c28529ceb5
Successfully tagged test:test
As you can see, diff is showing the differences before/after running sed and the modifications you are expecting are there.
We can also make sure those modifications persist when starting a container from this image:
$ docker run -it --rm --name testcmd test:test bash -c "grep -A 2 '\[system_default_sect\]' /etc/ssl/openssl.cnf"
[system_default_sect]
MinProtocol = TLSv1.0
CipherString = DEFAULT#SECLEVEL=1