Fix: Remove UTF-8 encoding to prevent mangling of binary secrets

[matthiasfehr]

This PR removes the explicit encoding: 'utf-8' from the secret resolution code to ensure binary files are not corrupted. Previously, reading and writing secrets as UTF-8 caused issues when the secret was a binary file. By switching to raw buffers, the code now correctly supports both binary and text-based secrets without any unintended transformations.

• Removed encoding: 'utf-8' from fs.readFileSync.
• Ensured raw buffers are used when reading and writing secrets, preventing data corruption.
• Maintains existing behavior for text-based secrets, while adding proper support for binary data.

[crazy-max]

This PR removes the explicit encoding: 'utf-8' from the secret resolution code to ensure binary files are not corrupted.

I was looking at the initial implementation and it was added in https://github.com/docker/build-push-action/pull/296/files#diff-f73f5f656aeb1aac9f0e62af243004b96603175d3d897231961ab4245d7a5a50R41

I think your change is fine but I just wonder what's your use case to have a bin formatted secret to use in your Dockerfile. Do you mind to share?

Also I think instead of doing both fs.readFileSync and fs.writeFileSync we could just do a fs.copyFileSync for file-based secrets like:

  public static resolveSecret(kvp: string, file: boolean): [string, string] {
    const [key, value] = Build.parseSecretKvp(kvp);
    const secretFile = Context.tmpName({tmpdir: Context.tmpDir()});
    if (file) {
      if (!fs.existsSync(value)) {
        throw new Error(`secret file ${value} not found`);
      }
      fs.copyFileSync(value, secretFile);
    } else {
      fs.writeFileSync(secretFile, value);
    }
    return [key, secretFile];
  }

[matthiasfehr]

I have a docker build stage in my Dockerfile where I sign the built executable, and for this I need to pass in a p12 certificate file. This works when directly passed as secret argument to docker but not with this action.

I have updated the PR to use directly the copyFileSync method.

[crazy-max]

Can you squash your commits? Otherwise LGTM thanks!