Share the encrypted .env file through git and the development private key through your team's approved private channel.
Retrieve the key:
$ dotenvx keypair DOTENV_PRIVATE_KEY
Your teammate puts the value in an ignored .env.keys file:
DOTENV_PRIVATE_KEY="your-shared-private-key"
They can then build the image and use the read-only key mount from Ship.
Keep .env.keys out of git and Docker images. Anyone retaining a copy of the key can still decrypt the file after repository access is removed.