chore: add provenance attestation to docker-base image (#14999)

Enables [build
attestation](https://docs.docker.com/build/metadata/attestations/slsa-provenance/)
for the docker-base image.
Contributes to #14879 and coder/internal#89

As an experiment, we are only doing it with the coder-base image for
now.

.github/workflows/docker-base.yaml

@@ -66,6 +66,7 @@ jobs:
           context: base-build-context
           file: scripts/Dockerfile.base
           platforms: linux/amd64,linux/arm64,linux/arm/v7
+          provenance: true
           pull: true
           no-cache: true
           push: ${{ github.event_name != 'pull_request' }}

.github/workflows/release.yaml

@@ -263,6 +263,7 @@ jobs:
           context: base-build-context
           file: scripts/Dockerfile.base
           platforms: linux/amd64,linux/arm64,linux/arm/v7
+          provenance: true
           pull: true
           no-cache: true
           push: true