Rebuild only code in docker - docker

Question

I have quite big yii2 application that takes a few minutes to build in docker. Is there a way to build code only, without "re-installing" everything each time? How do I speed up developing/debugging of dockerized yii2 app?
Now I do this:
docker build -t myapp:mytag .
docker run --name myapp -p 8000:8000 myapp:mytag
My Dockerfile:
FROM php:5.6-apache
COPY . /var/www/html/
ENV APACHE_DOCUMENT_ROOT /var/www/html/web
RUN sed -ri -e 's!/var/www/html!${APACHE_DOCUMENT_ROOT}!g' /etc/apache2/sites-available/*.conf
RUN sed -ri -e 's!/var/www/!${APACHE_DOCUMENT_ROOT}!g' /etc/apache2/apache2.conf /etc/apache2/conf-available/*.conf
RUN apt-get update && \
apt-get install -y curl nano unzip zlib1g-dev git && \
docker-php-ext-install pdo pdo_mysql zip && \
curl -sS https://getcomposer.org/installer | php -- --install-dir=/usr/local/bin --filename=composer
RUN cd /var/www/html && composer install
RUN cd /var/www/html/ && mkdir web/assets/
RUN chmod 777 /var/www/html/web/assets/
RUN mv /var/www/html/vendor/bower-asset/ /var/www/html/vendor/bower/

Answer 1

Docker will reused cached build steps that it has previously performed and have not changed. However, once you reach a step that breaks the cache, all subsequent steps have to rerun since the cache includes a dependency on the previous step. Therefore, the cache is order dependent and you have the following as one of your very first steps:
COPY . /var/www/html/
Every time you change your code, that line will have to be rerun, which then forces the apt-get lines to rerun as well. By reordering your install you'll see a large speed up:
FROM php:5.6-apache
ENV APACHE_DOCUMENT_ROOT /var/www/html/web
RUN sed -ri -e 's!/var/www/html!${APACHE_DOCUMENT_ROOT}!g' /etc/apache2/sites-available/*.conf
RUN sed -ri -e 's!/var/www/!${APACHE_DOCUMENT_ROOT}!g' /etc/apache2/apache2.conf /etc/apache2/conf-available/*.conf
RUN apt-get update && \
apt-get install -y curl nano unzip zlib1g-dev git && \
docker-php-ext-install pdo pdo_mysql zip && \
curl -sS https://getcomposer.org/installer | php -- --install-dir=/usr/local/bin --filename=composer
# move this line to as late in the build as possible
COPY . /var/www/html/
RUN cd /var/www/html && composer install
RUN cd /var/www/html/ && mkdir web/assets/
RUN chmod 777 /var/www/html/web/assets/
RUN mv /var/www/html/vendor/bower-asset/ /var/www/html/vendor/bower/