eventing/samples: some message_dumper images in samples aren't tagged

[ahmetb]

(reporting when master is at commit dfc83fe)

Looks like some message_dumper images have a @sha256:[...] tag which is nice and a comment indicating where they come from.

Ideally they all should be the same. So I'm wondering if the ones with the tag are created through some automation. (If so, we should apply that automation to all samples.)

$ ag -B 1 'gcr.io/knative-releases/github.com/knative/eventing-sources/cmd/message_dumper'
github-source/README.md
41-          container:
42:            image: gcr.io/knative-releases/github.com/knative/eventing-sources/cmd/message_dumper

github-source/service.yaml
11-          container:
12:            image: gcr.io/knative-releases/github.com/knative/eventing-sources/cmd/message_dumper

cronjob-source/README.md
29-          container:
30:            image: gcr.io/knative-releases/github.com/knative/eventing-sources/cmd/message_dumper

cronjob-source/service.yaml
12-          container:
13:            image: gcr.io/knative-releases/github.com/knative/eventing-sources/cmd/message_dumper

gcp-pubsub-source/subscriber.yaml
14-            # https://github.com/knative/eventing-sources/blob/release-0.3/cmd/message_dumper/dumper.go
15:            image: gcr.io/knative-releases/github.com/knative/eventing-sources/cmd/message_dumper@sha256:8423232db7a7b4010c0cfbfaef95745efe962631af9b7456903825801a7893f7

kubernetes-event-source/subscription.yaml
30-          container:
31:            image: gcr.io/knative-releases/github.com/knative/eventing-sources/cmd/message_dumper@sha256:73a95b05b5b937544af7c514c3116479fa5b6acf7771604b313cfc1587bf0940