git show command not giving the correct output when using a docker image

I wrote a bash script to run a branch cleanup for a repository, which will consider the branches which are not used in 3 months and send an email to the branch owner. In the script I wrote there is a small function which gets the branch name and the last committed date using git commands. Tis is how it looks like,

get_active_branches(){
    declare -ag active_branches=()

    echo -e "e[1;32m Scanning all active branches... e[0m"

    for branch in `git branch -r | grep -v HEAD`; do 
        branch_data=$(git show --format=%cs "$branch" | head -n 1)
        active_branches+=("$branch_data $branch")
    done 

    echo -e "e[1;32m Done. e[0m"
}

I am running this in a docker image, created using osixia/openldap as the base image. I’m using that image because I need to have access to an active directory in a ldap server to get the email address by providing the username of a developer.

The issue is that when I run the above function in a bitbucket pipeline, for the command branch_data=$(git show --format=%cs "$branch" | head -n 1) the branch_data variable get the value as %cs, for an example,

git show --format=%cs master
 branch_data=%cs

The Dockerfile I’m using is lokks like this,

FROM osixia/openldap

RUN apt-get update && apt-get install -y tar xz-utils git openssl bash curl jq gcc libc-dev vim coreutils && 
    rm -rf /var/lib/apt/lists/*

# Install kubectl
RUN apt-get update && apt-get install -y util-linux bash fortune sed findutils openssh-client git jq openssl curl && 
    curl -L https://storage.googleapis.com/kubernetes-release/release/v1.21.0/bin/linux/amd64/kubectl > /usr/bin/kubectl && 
    chmod +x /usr/bin/kubectl 

# Add actual scripts to /bin
ADD bb_cleanup_new.sh /bin/

bb_cleanup_new.sh is the script file which has the function.

Any suggestions to fix this issue.

I tested the script locally and everything works fine.
I thought maybe it’s the issue with the branch name having the prefix of origin/, so I tested after removing the prefix and running a bitbucket pipeline but it still gives the same result.
It only gives the issue when I’m using the image.