test-in-docker: quickly test frameworks in docker

This adds `./test-in-docker` for quickly playing with various frameworks.

All the containers are based off Ubuntu 14.04 which has ZSH 5.0.2.

Thanks to @dritter for figuring out all the framework installation
methods.

docker/zplugin/Dockerfile

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+FROM p9k:base
+
+COPY docker/zplugin/install.zsh /tmp/
+RUN zsh /tmp/install.zsh
+
+COPY ./ p9k/
+COPY docker/zplugin/zshrc.plugins .zshrc.plugins

docker/zplugin/install.zsh

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+sh -c "$(curl -fsSL https://raw.githubusercontent.com/psprint/zplugin/master/doc/install.sh)"
+
+# The 'zplugin snippet' only copies the .zsh-theme file, not everything else.
+mkdir -p ~/.zplugin/snippets
+ln -nsf \
+  ~/p9k/ \
+~/.zplugin/snippets/--SLASH--home--SLASH--fred--SLASH--p9k--SLASH--powerlevel9k--DOT--zsh-theme
+
+{
+  echo
+  echo "source ~/.zshrc.plugins"
+} >> ~/.zshrc

docker/zplugin/zshrc.plugins

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+#!zsh
+
+zplugin load psprint zsh-navigation-tools
+zplugin load psprint---zprompts
+zplugin snippet ~/p9k/powerlevel9k.zsh-theme