…aths (#774)

Fixes #773

Resolves #770

Related to #782 and #444 

- Allows concurrent decryption of different secrets files
- Caches decrypted secrets by original file path and returns decrypted results from memory
- Secrets being run through an instance of helmexec will be cached and run as fast as possible concurrently

NB: This particular PR doesn't make _all_ calls to secrets cached and concurrent.  Environment Secrets in particular seem to not be evaluated with a ScatterGather(), and doesn't use the same helmexec instance as other parts of the code, so it doesn't take advantage of these changes.  Some reworking of the plumbing there would be needed.

Fixes #793

* Add kubectl and jq to Dockerfile

Signed-off-by: Jake Hill <jake@naphta.uk>

* Update kubectl to use checksum verification, and pin to a specific version

Signed-off-by: Jake Hill <jake@naphta.uk>

Resolves #792

Ref #782

Closes #444 and #782 

This is the final PR to fully cache and parallelize helm secret decryption.  It threads the shared helmexec.Interface into the StateCreator and HelmState structs to be used during environment secret decryption.  This should effectively cache secrets for the duration of a helmfile run, regardless of where they are first decrypted.

Fixes #815

fix: clean up invalid remote state file cache

So that the release process can be (probably) 2x faster

#821)"

This reverts commit ba0891b.

Please notice me if anyone is using Helmfile on it

1. Added `helmfile build` command to print final state
Motivation: useful for debugging purposes and some CI scenarios

Ref #780 

2. Template interpolation is now recursive (you can cross-reference release fields) like:
```yaml
templates:
  release:
    name: {{`app-{{ .Release.Namespace }}`}}
    namespace: {{`{{ .Release.Labels.ns }}`}}
    labels:
      ns: dev
```
3. Experimental: Added some boolean release fields interpolation in templates:
```yaml
templates:
  release:
    name: {{`app-{{ .Release.Namespace }}`}}
    namespace: dev
    installedTemplate: {{`{{ eq .Release.Namespace "dev" }}`}}
```

Resolves #818

4. Added more template interpolations: Labels, SetValues
5. Added template interpolation for inline Values
6. Added `helmfile list` command to print target releases in simple tabular form
7. Added release names in some `helm` output messages, e.g.: `Comparing release=%v, chart=%v`

This is a follow-up for #823
Ref #823 (comment)