Push `REPO_VERSION` var into containers for builds #237

Job	Run time
Makefile / Assert PR branch is ahead	6s
CI matrix / Assert PR branch is ahead	5s
Makefile / Build codebase using Makefile quick recipe	25s
Makefile / Build codebase using Makefile all recipe	0s
Makefile / Lint codebase using Makefile	0s
CI matrix / Run tests (go-ci-oldstable, false)	20s
CI matrix / Run tests (go-ci-stable, false)	24s
CI matrix / Evaluate loopvar scoping changes	25s
CI matrix / Build codebase (go-ci-oldstable, false)	24s
CI matrix / Run tests (go-ci-unstable, true)	25s
CI matrix / Build codebase (go-ci-stable, false)	22s
CI matrix / Build codebase (go-ci-unstable, true)	22s
CI matrix / Lint codebase (go-ci-oldstable, false)	26s
CI matrix / Lint codebase (go-ci-stable, false)	26s
CI matrix / Lint codebase (go-ci-unstable, true)	24s
CI matrix / Optional Linting / Assert PR branch is ahead	8s
CI matrix / Optional Linting / Lint codebase using optional linters (go-ci-oldstable, true)	23s
CI matrix / Optional Linting / Lint codebase using optional linters (go-ci-stable, true)	24s
CI matrix / Optional Linting / Lint codebase using optional linters (go-ci-unstable, true)	25s
	5m 54s

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