refactor(e2e): use consistent tag name when building images at the same time

[TheSpiritXIII]

tl;dr: this ensures that when you run make e2e, all Docker images are built with the same tag.

Another try for #845 (thanks @pintohutch)

How this works

1. The problem is we call $(MAKE) which calls and forks make without carrying variables over. This is fixed by using export.
2. The second problem is that make has rules for variable expansion. = and ?= are considered "recursively expansions" which you can think of as lazily evaluated. := is considered "simply expanded" and is evaluated at declaration time. This is what we want to ensure it's only evaluated once.
3. Now that we export the value correctly to targets, we need to make sure we don't override it, so we add the conditional.
4. We still use the latest tag for e2e tests because we need this for the e2e-only target which does not rebuild all images. This needs to be conditional so I'll fix this in a separate PR.
5. I added nanoseconds to the tag which helps with testing but also better so we reduce the odds of mutating tags for older images.

[bernot-dev]

Added some suggestions.

[bernot-dev]

Can we just do this?

BUILD_DATE:=$(shell date "+%Y%d%m_%H%M%S_%N")
export TAG_NAME?=gmp-${BUILD_DATE}

[TheSpiritXIII]

I believe so. They point out it is equivalent in: https://www.gnu.org/software/make/manual/html_node/Setting.html

[TheSpiritXIII]

I investigated this and they're not 1:1.

With yours, we will always evaluate BUILD_TIME. If someone uses it and calls $(MAKE), the value will be different in each execution in each target.

The reason why mine works is because my if statement says if the variable is not set, then set it. In your example, TAG_NAME is lazily evaluated which also means you also can't simply as export TAG_NAME?=gmp-$(shell date "+%Y%d%m_%H%M%S_%N"), since it's re-evaluated each time you call $(MAKE). While your version is simpler I feel it's more error-prone in these 2 ways.

As such, I kept my original version. Any strong thoughts here?

[bernot-dev]

Suggestion: It's possible to do multiple tags with one build command. No need to call tag separately for this case.

[TheSpiritXIII]

That's docker build. docker tag does not support multiple tags.