Updated Dockerfile reference and updated Dockerfile to match guidance

[andyloughran]

Updated the Dockerfile reference documentation as needed a refresh.

Referencing the OCI so that we can link back to the open standards that we're following, and the reason for following a particular implementation of the open standard.

Updated our ~/Dockerfile to match our guidance regarding sha pinning.

Guidance has changed

[galund]

I feel I don't know how I'd go about updating the Dockerfile, but that's a me problem not necessarily something for the GDS Way to fix. It might want to be in the readme for the repo, but I guess if it's the same in all cases then maybe not.

[andyloughran]

I feel I don't know how I'd go about updating the Dockerfile, but that's a me problem not necessarily something for the GDS Way to fix. It might want to be in the readme for the repo, but I guess if it's the same in all cases then maybe not.

Do you want me to document how to update the sha of a Dockerfile based on the comment above the FROM? I'll happily do that and I think you're right - it's implicit but it should be more explicit.

[willp-bl]

I feel I don't know how I'd go about updating the Dockerfile, but that's a me problem not necessarily something for the GDS Way to fix. It might want to be in the readme for the repo, but I guess if it's the same in all cases then maybe not.

Do you want me to document how to update the sha of a Dockerfile based on the comment above the FROM? I'll happily do that and I think you're right - it's implicit but it should be more explicit.

That will be a good addition. Once that is done is this PR ready to be merged?