Reach js-conflux-sdk git repo cannot be read when building from CI Docker container

The step in my Docker file which reads RUN yarn install --frozen-lockfile is failing with the following error text when executed in a .github action.

It reads as though there's a problem running git ls remote --tags --heads ssh://git@github.com/reach-sh/js-conflux-sdk.git which further seems to indicate that 'ssh: not found'

'ssh' is definitely present within the build environment, and the reach-sh/js-conflux-sdk repo is not private. I cannot reproduce this on my local machine. What am I missing?

#10 [builder 6/7] RUN yarn install
#10 sha256:0241e5ad2b01044255a03052e8f2a2540c5fb45a447a1b93c55b90ed86436440
#10 10.84 yarn install v1.22.17
#10 11.81 [1/4] Resolving packages...
#10 12.75 [2/4] Fetching packages...
#10 13.13 error Command failed.
#10 13.13 Exit code: 128
#10 13.13 Command: git
#10 13.13 Arguments: ls-remote --tags --heads ssh://git@github.com/reach-sh/js-conflux-sdk.git
#10 13.13 Directory: /apps/launchpad-api
#10 13.13 Output:
#10 13.13 "ssh" -oBatchMode=yes: line 1: ssh: not found
#10 13.13 fatal: Could not read from remote repository.
#10 13.13 
#10 13.13 Please make sure you have the correct access rights
#10 13.13 and the repository exists.
#10 13.13 info Visit https://yarnpkg.com/en/docs/cli/install for documentation about this command.
#10 ERROR: executor failed running [/bin/sh -c yarn install]: exit code: 128
------
 > [builder 6/7] RUN yarn install:
------
executor failed running [/bin/sh -c yarn install]: exit code: 128

I've attempted explicitly installing ssh into the build container which reported that ssh was already installed.

I've checked that git ls-remote --tags --heads ssh://git@github.com/reach-sh/js-conflux-sdk.git can be run from my local machine without any special permissions.

I've tried to delete and rebuild my yarn.lock file which didn't do anything at all differently.