beaver install

You should move to use a beaver.Dockerfile unless you really need some
customization that is not supported by beaver docker gen-dockerfile. Please
read the New Features notes to see how to use this.
beaver dlang install

This command use a much more automated approach to building the image, by
using a beaver.Dockerfile and docker/build script (like beaver install).

Note that, on the contrary to beaver install this command won't fall
back to use a Dockerfile.$DIST, so you need to migrate to the new scheme.

Features

beaver docker gen-dockerfile

The docker command learned a new subcommand to generate a Dockerfile based
on a beaver.Dockerfile (by default). Please look read beaver docker gen-dockerfile -h for more options and information on how the file is
actually generated.
beaver dlang install

This command, as beaver install, learned to use the beaver.Dockerfile to
generate the Dockerfile. Please read the changes in beaver install to
learn how to use this feature.

This command works exactly like beaver install but it adds more sugar on
top. It will install the DMD version as requested via the DMD environment
variable (by injecting the DMD_PKG variable to the docker image generation,
as well as calling apt-get update && apt-get install with the requested DMD
version for you.

This means you only have to take care of installing your project dependencies
and you don't need to do an apt-get update before the install.

Here is an example beaver.Dockerfile and docker/build script when using
beaver dlang install:
```
FROM sociomantictsunami/dlang:v2
```
```
#!/bin/sh
apt-get install -f libwhatever-dev tool
```
Again, if you don't need extra dependencies, you can completely omit the
docker/build script.

Note that beaver dlang install won't fall back to use
a Dockerfile.$DIST.
beaver install

This command learned to use the beaver.Dockerfile to generate the
Dockerfile. The Dockerfile.$DIST still has precedence over the
beaver.Dockerfile in case you really need to customize the Dockerfile in
a way that beaver docker gen-dockerfile is not enough.

By using a beaver.Dockerfile you can just use one docker image building
specification for multiple base distributions. Your beaver.Dockerfile only
needs to define a FROM line in a special way, only using as the tag, the
image version without the distribution name.

So if you want to use sociomantictsunami/dlang:{trusty,xenial}-v2, you
should use:
```
FROM sociomantictsunami/dlang:v2
```
Of course you can use other Dockerfile instructions here as needed.

The beaver install command will automatically inject the appropriate
distro name and some final instructions to copy everything in the docker/
directory to the image and run the docker/build script so you just need to
care to write a script to build the image (make sure it is executable). In the
script you can always use $(lsb_release -cs) to the what the current Ubuntu
version is and install different packages based on that, for example.

Here is a sample script:
```
#!/bin/sh
set -xeu

apt-get update

if test "$(lsb_release -cs)" = trusty
then
    apt-get install -y python2
else
    apt-get install -y python3
fi
```
You can completely omit the docker/build script if you don't need any extra
build steps - if the script doesn't exist, nothing will be copied to the image
building procedure either.