add ca-certificates to light docker image

[atheiman]

I found that when hashicorp/consul-template:light tries to talk to my Vault server that is secured with a Let’s Encrypt cert, it fails. (vault fqdn changed to vault.example.com in logs)

2019/03/20 18:13:47.270390 [WARN] (view) vault.read(secret/mydatabase): vault.read(secret/data/database): Get https://vault.example.com/v1/sys/internal/ui/mounts/secret/data/database: x509: certificate signed by unknown authority (retry attempt 1 after "250ms")
2019/03/20 18:13:47.398900 [WARN] vault.token: failed to renew: Put https://vault.example.com/v1/auth/token/renew-self: x509: certificate signed by unknown authority

If I run apk add --no-cache ca-certificates before running consul-template, the problem is fixed.

2019/03/20 18:14:22.207209 [INFO] (runner) rendered "./system.properties.ctmpl" => "./system.properties"

[atheiman]

@pearkes thanks for your help when I added this light image in #1191 - could you help me out with reviewing this one?

[atheiman]

anyone that can help with this?

[eikenb]

Hey @atheiman, thanks for the contribution. I'll be looking at this when putting together my initial, "hey look a new maintainer", release.