Docker guidelines

# Guidelines
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###### 1. `FROM` command first
the `FROM` command must be the first to appear in a dockerfile

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###### 2. `RUN` Exec form
`RUN` commands have two syntaxes, one with brackets and one without. Interpretation of arguments differ from the two syntaxes. The one with brackets must be used.

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###### 3. Multiple `CMD`
`CMD` commands allows one to start a service when booting up a container. Docker allows only **a single service** to be specified, therefore multiple `CMD` are useless since only the last one will be run.

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###### 4. Provides default to `CMD`
One has to provide default parametervia `CMD` to start a service. If an `EntryPoint` command is specified, `CMD` and `EntryPoint` commands should be specified in `JSON` format.

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###### 5. Variables in exec form of `CMD`
Variables used in `CMD` commands in its exec form are not interpreted.
`CMD [ "echo", "$HOME" ]` won't output the `$HOME` variable value.

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###### 6. Merge `LABEL` commands
When possible, merge labels commands.

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###### 7. Avoid `apt-get upgrade`
You should avoid `RUN apt-get upgrade` or `dist-upgrade`, as many of the “essential” packages from the base images won’t upgrade inside an unprivileged container

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###### 8. Combine `install` with `update`

Always combine `RUN apt-get update` with `apt-get install` in the same `RUN` statement. Ommiting this can lead to unexpected behaviour since `apt-get update` can be not run.

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###### 9. Packages, version pinning

Always fully specify the version of the package to install.

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###### 10. `FROM`, version pinning

Always fully specify the version of the parent dockerfile to use (**i.e.** __latest__ tag is therefore not permitted).

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###### 11. `CMD` exec form
`CMD` commands have two syntaxes, one with brackets and one without. Interpretation of arguments differ from the two syntaxes. The one with brackets must be used if parameters are specified.

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###### 12. Prefer `COPY` 
Although `ADD` and `COPY` are functionally similar, generally speaking, `COPY` is preferred.

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###### 13. `ADD <http>` discouraged
Because image size matters, using `ADD` to fetch packages from remote `URL`s is strongly discouraged; you should use `curl` or `wget`.

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###### 14. User `root` discouraged
You should avoid installing or using `sudo` since it has unpredictable. `TTY` and signal-forwarding behavior that can cause more problems than it solves. If you absolutely need functionality similar to `sudo` (e.g., initializing the daemon as `root` but running it as non-root), you may be able to use `gosu`.

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###### 15. Less `USER` commands as possible
To reduce layers and complexity, avoid switching `USER` back and forth frequently.

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###### 16. `WORKDIR` must have absolute path 
For clarity and reliability, you should always use absolute paths for your `WORKDIR`.

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###### 17. `cd` in `RUN` should be avoided
Don’t use `cd` in `RUN` commands, use `WORKDIR` instead.

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###### 18. Sort installation alphanumerically
Installation of multiple softwares must be written in alphanumerical order.

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###### 19. Add `–no-install-recommend`
Add `–no-install-recommend` when installing with `apt-get`, this will avoid installation not explicitly specified.