Dockerfile - Load key "/root/.ssh/id_rsa": invalid format with Build Kit

I got the following error:

11 2.895 In Git.php line 453:
#11 2.895                                                                                
#11 2.895   Failed to execute git clone --mirror -- 'git@gitlab.com:test/test/  
#11 2.895   test/data-etstgit' '/root/.composer/cache/vcs/git-gitlab.com
#11 2.895   -test.git/'                                 
#11 2.895                                                                                
#11 2.895   Cloning into bare repository '/root/.composer/cache/vcs/git-gitlab.com
#11 2.895  test.git'...                               
#11 2.895   Warning: Permanently added the RSA host key for IP address '1.1.11.1' to   
#11 2.895   the list of known hosts.                                                     
#11 2.895   Load key "/root/.ssh/id_rsa": invalid format                                 
#11 2.895   git@gitlab.com: Permission denied (publickey).                          
#11 2.896   fatal: Could not read from remote repository.                                
#11 2.896                                                                                
#11 2.896   Please make sure you have the correct access rights                          
#11 2.896   and the repository exists.  

While building this test Dockerfile:

FROM alpine

RUN mkdir /root/.ssh/
RUN --mount=type=secret,id=ssh_key,dst=/etc/secrets/ssh_key cat /etc/secrets/ssh_key > /root/.ssh/id_rsa && chmod 400 /root/.ssh/id_rsa
RUN cat /root/.ssh/id_rsa

RUN touch /root/.ssh/known_hosts && \
      ssh-keyscan -t rsa gitlab.com >> /root/.ssh/known_hosts 

ADD source .
RUN composer install -o -n --no-scripts

RUN mkdir -pv -m 0777 var var/log var/cache var/cache/prod && \
      chmod -R 777 var && chown -R 33:33 var

The command that I execute for it:

echo -n "$BITBUCKET_SSH_KEY" > ssh_key
    DOCKER_BUILDKIT=1 docker build --pull --secret id=ssh_key,src=./ssh_key -f Dockerfile-php -t ${TAG} .

However, I have also tried just to use env varible for docker secret with the following build command:

DOCKER_BUILDKIT=1 docker build --pull --secret id=SSH_KEY,env=BITBUCKET_SSH_KEY -f Dockerfile-php -t${TAG} .

And with this changed Dockerfile layer:

RUN --mount=type=secret,id=SSH_KEY cat /run/secrets/SSH_KEY  > /root/.ssh/id_rsa && chmod 400 /root/.ssh/id_rsa

With debug layer - RUN cat /root/.ssh/id_rsa I see that id_rsa looks totally fine inside container, as it should look like ssh key, however the error of invalid format still persists

Start with Dockerfile-php and the supplied test Dockerfile, then reproduce the issue using the --mount=type=secret Docker BuildKit commands and both file- and environment-backed secrets. Trace how the mounted key reaches /root/.ssh/id_rsa and compare it with the SSH failure; done means identifying the cause of the invalid-format error and documenting or validating the relevant fix.