Add checks for:
* Docker installed locally
* Docker buildx plugin installed locally
* Dockerfile exists
If checks fail, it will halt deployment and provide more specific error messages.
Also adds a cli subcommand:
`mrsk build dependencies`
Fixes: #109 and #237
If we get an error we'll only hold the deploy lock if it occurs while
trying to switch the running containers.
We'll also move tagging the latest image from when the image is pulled
to just before the container switch. This ensures that earlier errors
don't leave the hosts with an updated latest tag while still running the
older version.
* `-e [REDACTED]` → `-e SOME_SECRET=[REDACTED]`
* Replaces `Utils.redact` with `Utils.sensitive` to clarify that we're
indicating redactability, not actually performing redaction.
* Redacts from YAML output, including `mrsk config` (fixes#96)
`docker image ls` doesn't tell us what the latest deployed image is (e.g
if we've rolled back). Pull the latest image tag through to the server
so we can use it instead.
