Cannot clone a private Bitbucket repo from inside a Docker image using docker-compose

I am trying to clone a private Git repo hosted on Bitbucket Cloud from inside a docker image. I build the image using docker-compose (ver 3.9).

I have added the public key as an Access Key in the Repo settings in Bitbucket. 

Here is the error I get:

 => ERROR [16/19] RUN git clone git@bitbucket.org:some_repo/imp_hmi.git 0.7s
------ 
> [16/19] RUN git clone git@bitbucket.org:some_repo/imp_hmi.git:
#0 0.331 Cloning into 'imp_hmi'...
#0 0.636 Host key verification failed.
#0 0.636 fatal: Could not read from remote repository.
#0 0.636 
#0 0.636 Please make sure you have the correct access rights
#0 0.636 and the repository exists.

I can clone the repo using the same SSH keys on the host machine.

Now, for the Dockerfile:

# Update this value when the version changes.
ARG UNITY_VERSION=2020.3.13f1
#ARG HMI_CONFIG=niro_av71oxu.yaml
FROM unityci/editor:ubuntu-${UNITY_VERSION}-linux-il2cpp-1.0.1 AS base

USER root
ENV HOME /home/root
# # don't ask interactive questions
ENV DEBIAN_FRONTEND noninteractive

# Create user bobsaccamano
RUN useradd -m -r bobsaccamano
RUN usermod -aG adm,cdrom,sudo,audio,dip,video,plugdev bobsaccamano

# Setup SSH keys
RUN mkdir -p -m 0700 /home/bobsaccamano/.ssh
COPY id-docker-unity /home/bobsaccamano/.ssh/
RUN chown bobsaccamano:bobsaccamano /home/bobsaccamano/.ssh/id-docker-unity
RUN chmod 600 /home/bobsaccamano/.ssh/id-docker-unity
COPY id-docker-unity.pub /home/bobsaccamano/.ssh/ 
RUN chown bobsaccamano:bobsaccamano /home/bobsaccamano/.ssh/id-docker-unity.pub
RUN touch /home/bobsaccamano/.ssh/known_hosts && chown bobsaccamano:bobsaccamano /home/bobsaccamano/.ssh/known_hosts
RUN ssh-keyscan bitbucket.org >> /home/bobsaccamano/.ssh/known_hosts
RUN cat /home/bobsaccamano/.ssh/id-docker-unity

# Change to bobsaccamano user
USER bobsaccamano
ENV HOME /home/bobsaccamano
ENV HMI_BUILT ${HOME}/HMI_built

# Create folders
RUN mkdir -p -m 0700 /home/bobsaccamano/proj/
RUN mkdir -p -m 0700 ${HMI_BUILT}

# Pull Repositories
WORKDIR /home/bobsaccamano/proj/
RUN git clone git@bitbucket.org:some_repo/imp_hmi.git

# Build HMI
RUN cd imp_hmi && chmod +x build_hmi.sh
RUN . build_hmi.sh DEV

WORKDIR ${HOME}
#RUN apt-get -y update
# WORKDIR /home/unity_volume

The docker-compose.yml file:

version: "3.9"
services:
unity_base:
build:
context: .
dockerfile: Dockerfile.unity
# args:
# progress: plain
volumes:
- hmi_built:/home/bobsaccamano/HMI_built
container_name: unity-base
hmi_app:
build:
context: .
dockerfile: Dockerfile.hmi
depends_on:
- unity_base
volumes:
- hmi_built:/home/bobsaccamano/HMI_built
container_name: hmi-app
volumes:
hmi_built:

Any help is much appreciated!