Derive Dockerfile instruction names from pkg/docker/instruction instead of duplicating them

[Londopy]

pkg/docker/dockerfile/dockerfile.go and pkg/docker/dockerfile/reverse/reverse.go each carried their own uppercase copies of the Dockerfile instruction names (instTypeCmd = "CMD", instPrefixCmd = "CMD ", ...) alongside the canonical lowercase set in pkg/docker/instruction.

This replaces both const blocks with var blocks that derive the uppercase form from instruction.* (strings.ToUpper(instruction.Cmd)), so there's one set of names to maintain and the two files can't drift from it. The lowercase constants stay canonical because the AST parser, parser, and linter checks all key on them; the uppercase form is still needed here for image-history matching and Dockerfile output (InstructionInfo.Type is serialized uppercase, so no output change).

The local identifiers are kept, so none of the ~25 usage sites change. dockerfile.go only ever used the USER/EXPOSE/WORKDIR prefixes, so the other 17 unused names there are just dropped.

Verified with GOOS=linux go build ./pkg/docker/..., go vet, and the reverse package tests (run on Linux).

Not touched, in case you want them as a follow-up: the inline fmt.Sprintf("RUN %s", ...) / "EXPOSE %s" / WriteString("ENV ") literals in the same two files.

Closes #74