--single-snapshot fails if there is a MAINTAINER command in the dockerfile

[tomprince]

Actual behavior
If there is a MAINTAINER command in a dockerfile, and --single-snapshot is passed, no snapshot of the stage with MAINTAINER is taken (and the final image from that stage is identical to the from image)

Expected behavior
A snapshot it taken after all the commands run.

Additional Information

• Dockerfile

FROM scratch
MAINTAINER nobody@domain.test
ENV VAR=VAL

• Kaniko Image (fully qualified with digest): gcr.io/kaniko-project/executor:v0.19.0@sha256:66be3f60f22b571faa82e0aaeb94731217ba0c58ac4a3b062bc84c6d8d545213

Diagnosis:

• https://github.com/GoogleContainerTools/kaniko/blob/v0.19.0/pkg/executor/build.go#L221 iterates over stageBuilder.cmds
• pass the index to https://github.com/GoogleContainerTools/kaniko/blob/v0.19.0/pkg/executor/build.go#L417 which compares the index with the length of stageBuilder.stage.Commands
• since https://github.com/GoogleContainerTools/kaniko/blob/v0.19.0/pkg/commands/commands.go#L94 causes MAINTAINER to be skipped by stageBuilder.cmds but not stageBuilder.stage.Commands, isLastCommand is never true.

Triage Notes for the Maintainers

Description	Yes/No
Please check if this a new feature you are proposing
Please check if the build works in docker but not in kaniko
Please check if this error is seen when you use --cache flag
Please check if your dockerfile is a multistage dockerfile