Reduce number of Docker layers

[PuKoren]

• Use only 1 layer for apk installs
• Use only 1 layer for env vars set
• Describe shared volume required to run in Dockerfile (/migrations) with VOLUME instruction
• Small doc on readme for usage sample (with container name/tag)

[mbucc]

@synchrone Any comments? (@PuKoren I'm a docker noob, Synchrone is the one that started docker with shmig.)

[mbucc]

For my edification, from http://stackoverflow.com/a/33836848/1789168:

• a layer, or image layer is a change on an image, or an intermediate image.
• Every command you specify (FROM, RUN, COPY, etc.) in your Dockerfile causes the previous image to change, thus creating a new layer. [...]
• The concept of layers comes in handy at the time of building images.
• Because layers are intermediate images, if you make a change to your Dockerfile, docker will build only the layer that was changed and the ones after that.
• This is called layer caching.

[PuKoren]

After a check on my PR, it seems that I have moved default folder for migration from /sql to /migrations, should I update the PR to rollback to /sql?

[mbucc]

Yes, that change would break others that rely on that default.

-----Original Message----- From: "Jonathan Muller" notifications@github.com Sent: ‎2/‎13/‎2017 11:18 To: "mbucc/shmig" shmig@noreply.github.com Cc: "mbucc" mkbucc@gmail.com; "Comment" comment@noreply.github.com Subject: Re: [mbucc/shmig] Reduce number of Docker layers (#20)

After a check on my PR, it seems that I have moved default folder for migration from /sql to /migrations, should I update the PR to rollback to /sql? — You are receiving this because you commented. Reply to this email directly, view it on GitHub, or mute the thread.

[PuKoren]

You are right, updated my commit and doc

[PuKoren]

Updated with review comments, thanks

[synchrone]

LGTM