Alpine-based docker image

[ikedam]

Replaces #604 .

I want alpine-based docker images for sops, as it's small and easy to use:

• The current image uses 2 GB+:

  $ docker images mozilla/sops
  REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
  mozilla/sops        latest              6a8e325d2d11        5 weeks ago         2.3GB
  

• The alpine-based image uses only 65MB:

  $ docker build -t sops -f Dockerfile.alpine .
  ...
  $ docker images sops
  REPOSITORY          TAG                 IMAGE ID            CREATED              SIZE
  sops                latest              c4f6175388ce        About a minute ago   64.4MB
  

And this pull request also changes .circleci/config.yml to deploy alpine based images as mozilla/sops:3.5.0-alpine and mozilla/sops:alpine.
I believe we don't need alpine images for every commits and only images for tags are enough useful.

[ikedam]

I need to use workflow to have circleci run also for tags.

[codecov-io]

Codecov Report

Merging #609 into develop will increase coverage by 0.17%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop     #609      +/-   ##
===========================================
+ Coverage    36.98%   37.15%   +0.17%     
===========================================
  Files           21       21              
  Lines         2893     2893              
===========================================
+ Hits          1070     1075       +5     
+ Misses        1728     1724       -4     
+ Partials        95       94       -1

Impacted Files	Coverage Δ
stores/flatten.go	91.52% <0%> (+4.23%)	⬆️

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[autrilla]

LGTM mostly, but I think you should probably merge this into #607

[ikedam]

@autrilla Thank you, I rebased this to the latest develop and reworked based on #607.

[autrilla]

Thanks for working on this :)

[ikedam]

Thanks!