fix: linked/depends-on container restarting

[piksel]

Restarting of dependency containers has been something that has seemed broken in watchtower for some time, but there hasn't been any easy way to reproduce/verify it.
So, the first part of this PR is an e2e test that actually tests whether the "implicit restarts" actually work:
https://asciinema.org/a/444312 (spoiler: they don't)

Unfortunately, I don't think running this test in CI is viable (or even possible), but for now it serves it's purpose and can be run locally when needed.

The second part is actually fixing the problem (mutating the actual array struct instead of a copy):
https://asciinema.org/a/444307

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The test script creates two images which run a small webserver that only returns the image version number, and pushes them to a local docker registry (which can be created using scripts/du-cli.sh registry start). Then it creates two containers, with one of them being dependent on the other (through depends-on label or --linked) using their corresponding images.
Next, it creates another revision of the "parent" containers image and pushes it to the local registry and runs watchtower.
After watchtower has ran, it queries the two containers to see if the start time has changed (meaning that the container was at least restarted), and if the version number returned from the service inside the containers have changed (meaning that the container was updated).

Test script usage:

./scripts/dependency-test.sh <linked | label> [...watchtower args]

This will run watchtower by building it from the source. To instead run a released image in docker, set the environment variable WATCHTOWER_TAG to latest or latest-dev.
Example:

WATCHTOWER_TAG=latest ./scripts/dependency-test.sh linked

The first argument is whether the containers should be linked using --link or --label. All other arguments will be passed to the watchtower executable/container.

[codecov]

Codecov Report

Merging #1103 (f921a70) into main (4a66a69) will increase coverage by 0.47%.
The diff coverage is 91.66%.

@@            Coverage Diff             @@
##             main    #1103      +/-   ##
==========================================
+ Coverage   61.96%   62.43%   +0.47%     
==========================================
  Files          22       22              
  Lines        1438     1443       +5     
==========================================
+ Hits          891      901      +10     
+ Misses        463      458       -5     
  Partials       84       84              

Impacted Files	Coverage Δ
internal/actions/update.go	69.10% <91.66%> (+5.54%)	⬆️

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[jeff-h]

It would be fantastic if dependent container restarting was reliable! :)

Just to clarify though, is this fix so that:

• watchtower looks at docker-compose's depends_on (ie watchtower would need to look at all other containers and find any that depend on a restarting one)
• or is it for watchtower's com.centurylinklabs.watchtower.depends-on label, which as I understand it should be specified on the container being restarted?

Or either/both?

[piksel]

It's for --link and watchtowers depends-on label. There is no added support for docker compose.