chore: update ragel version used by CI

[wolffcm]

This was causing headaches because most users are using ragel 6.10 on
local machines, but the Docker image used 6.9 (several years old now).

Fixes #1183.

Done checklist

• docs/SPEC.md updated
• Test cases written

[wolffcm]

@nathanielc I was able to build and test this new Dockerfile_build locally and it seems okay, but CI is not picking it up. Do I need to update the image nathanielc/flux-build:latest in a Docker registry somewhere?

[nathanielc]

@wolffcm Yes, its a bit backwards, but the source of the image is the Dockerfile_build from the Flux repo, but the docker image CI uses will not be updated till the changes are merged to master. So its a bit of chicken and egg issue.

One way to solve would be to split this PR, one PR to change the docker image and one for the changes to the generated scanner. The docker image PR will go green, since it doesn't really change anything in CI. Then the second PR can be merged. The only catch is that no other PRs will go green until the changes to the scanner are merged.

[wolffcm]

@nathanielc Ok, makes sense. I made this PR just update the dockerfile, and created a separate PR for updating the generated code:
#1246

[nathanielc]

Why are gcc and libc-dev needed?

[wolffcm]

I found that go vet seems to use CGO, so it needs them. We wouldn't need them if we set CGO_ENABLED=0 in the environment. Do you have a preference?

[wolffcm]

I did a little more digging on this issue, I think that go vet needs to build something for some reason, and through transitive dependencies something that needs CGO (I seem to recall that the net package uses CGO) gets compiled.

This is a recent change in Go 1.11 that seems not to have been fixed in Go 1.12:
golang/go#26988

Maybe the best way forward would be to add CGO_ENABLED=0 to the environment, and if/when that Go bug gets fixed, we won't have any unneeded dependencies in the docker build.

[wolffcm]

As it turns out, it's not possible to use go test -race with an Alpine-based image:
golang/go#14481
Incidentally, go test -race also requires CGO, so we would need to include gcc and libc-dev dependencies anyway, but even with them there I get the link errors described in the golang issue.

I'll update the issue to reflect this, a different solution is needed.