Add CI/CD pipelines and deb/rpm packaging options

[ccremer]

What's included

• Adds Github Actions workflows to compile, test and build helm-docs within PRs and all branches
• Adds Goreleaser release automation that is only triggered upon push Git tags
• Adds Deb/Rpm packages to the release artifacts, so that users can install it using dpkg -i and the rpm equivalent. No additional dependency besides goreleaser is necessary for local development. The packages will install the binary in /usr/local/bin by default (see https://goreleaser.com/customization/#NFPM)
• Configures artifact checksum signing with GPG, so users can verify the downloads (instructions to set up below)
• Adds an additional Makefile target dist

Why

• GitHub Actions usage is free for public repositories
• It's faster and better documented than Travis CI
• Deb/Rpm packaging options gives better (un)installability and versioning for free (besides Docker)
• CI/CD improves code quality, it gives confidence when merging PRs
• Helm-docs is an awesome project :)

What you need to do before merging

Set up signing key

1. Create a new passwordless GPG key, e.g. gpg --quick-generate-key helm-docs default default never (Do NOT use your personal private key!)
2. Get the ID of the new key:
   gpg --list-keys, the id looks like E57761BCB035C49A7068EF97083880A377733DD7
3. Export the private key of the new GPG key for signing, e.g.
   gpg --armor --export-secret-key E57761BCB035C49A7068EF97083880A377733DD7
4. Save the content of the private key as a secret named SIGNING_KEY in https://github.com/norwoodj/helm-docs/settings/secrets (it begins with -----BEGIN PGP PRIVATE KEY BLOCK-----)
5. Check in the public key (gpg --armor --export E57761BCB035C49A7068EF97083880A377733DD7 > signature.asc) somewhere in this Repo, e.g. .github/signature.asc or in the project root dir, file name/extension also doesn't matter, it's just a proposal. So that users can verify the downloads with the public key.
6. Leave a comment in this PR with the GPG Id, so I can replace the placeholder in .goreleaser.yml:

signs:
- artifacts: checksum
  args: ["-u", "<thefingerprintid>", "--output", "${signature}", "--detach-sign", "${artifact}"]

(or you can do it directly in your own commit, I don't mind)

This only takes around 15min to do ;)

Set up CI/CD

1. Add a new secret named DOCKER_HUB_USER in https://github.com/norwoodj/helm-docs/settings/secrets (it's probably jnorwood like your Docker Hub repo, but for good measure, should it change somehow)
2. Create a new personal access token on Docker Hub (it's revocable in any case)
3. Store the token as DOCKER_HUB_PASSWORD in the secrets

(Note: Secrets do NOT get passed to PR builds or printed in CI output, so your Docker Hub access isn't leaked.)