buildah build: add a --env

Add a `--env` option to `buildah build` that functions similarly to the
`buildah config --env` option, to complement `buildah build`'s
`--unsetenv` option.

Document that `buildah config`'s `--env` function fetches the current
value for a variable when the name is supplied, but no `=` or value
follows it.

Signed-off-by: Nalin Dahyabhai <nalin@redhat.com>

commit.go

@@ -102,6 +102,7 @@ type CommitOptions struct {
 	// indexing. i.e. 0 is the first layer, -1 is the last (top-most) layer.
 	OciEncryptLayers *[]int
 	// UnsetEnvs is a list of environments to not add to final image.
+	// Deprecated: use UnsetEnv() before committing instead.
 	UnsetEnvs []string
 }