Add option to run deploy only if image update available

[arne-kroeger]

This commit adds an option to run the deploy only if the referenced image has an update from remote.

This can be used to minimize load if update not necessary or script is running in crons.

[c4710n]

This would be more solid by comparing docker config --hash output and com.docker.compose.config-hash container label.

Two related links:

• https://github.com/docker/compose-cli/issues/935#issuecomment-803314117
• https://github.com/docker/compose/issues/9211