We use `sops`[1] to do this and it works really well.
There is a Google Cloud KMS keyring (for typical usage) and a GPG key (for emergency/offline usage) set up to handle the encryption/decryption of files that store secrets for each application's deployment. I have some bash scripts that run on CI which are essentially just glorified wrappers to `sops` CLI to generate the appropriate `.env` file for the application, which is put into the container by the `Dockerfile`.
Applications are already configured to read configuration/secrets from a `.env` file (or YAML/JSON, depending on context), so this works pretty easily and avoids depending on secrets being set in the `ENV` at build time.
You can also, of course, pass any decrypted values from `sops` as arguments to your container deployment tool of choice (e.g. `helm deploy foo --set myapp.db.username=${decrypted_value_from_sops}`) and not bundle any secrets at build time at all.
There is a Google Cloud KMS keyring (for typical usage) and a GPG key (for emergency/offline usage) set up to handle the encryption/decryption of files that store secrets for each application's deployment. I have some bash scripts that run on CI which are essentially just glorified wrappers to `sops` CLI to generate the appropriate `.env` file for the application, which is put into the container by the `Dockerfile`.
Applications are already configured to read configuration/secrets from a `.env` file (or YAML/JSON, depending on context), so this works pretty easily and avoids depending on secrets being set in the `ENV` at build time.
You can also, of course, pass any decrypted values from `sops` as arguments to your container deployment tool of choice (e.g. `helm deploy foo --set myapp.db.username=${decrypted_value_from_sops}`) and not bundle any secrets at build time at all.
[1] https://github.com/mozilla/sops