make `oci_image()` accept a file for workdir #1566

Job	Run time
matrix-prep-os	1s
matrix-prep-bazelversion	2s
test (ubuntu-latest, 7.1.1, ., true)	54s
test (ubuntu-latest, 7.1.1, ., false)	6m 1s
test (ubuntu-latest, 7.1.1, e2e/wasm, false)	4m 18s
test (ubuntu-latest, 7.1.1, e2e/smoke, true)	2m 38s
test (ubuntu-latest, 7.1.1, e2e/smoke, false)	26s
test (ubuntu-latest, 7.1.1, e2e/assertion, true)	5m 8s
test (ubuntu-latest, 6.4.0, ., true)	3m 14s
test (ubuntu-latest, 6.4.0, ., false)	6m 16s
test (ubuntu-latest, 6.4.0, e2e/wasm, false)	3m 14s
test (ubuntu-latest, 6.4.0, e2e/smoke, true)	2m 27s
test (ubuntu-latest, 6.4.0, e2e/smoke, false)	21s
test (ubuntu-latest, 6.4.0, e2e/assertion, true)	4m 45s
	39m 45s

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