tao
Caching dependency compilation in Docker build
I’m deploying a Phoenix app with Elixir releases and Docker. The Phoenix docs have an example dockerfile you can use – mine is based on that one.
I’ve set up the file so that dependency downloads and compilation should be cached by Docker, but this isn’t happening. This is slowing down deploys a bit. Can anyone spot why Docker isn’t caching dependencies? Even when mix.exs is unchanged, it redownloads and rebuilds them.
FROM elixir:1.9.0-alpine as build
# install build dependencies
RUN apk add --update git build-base
# prepare build dir
RUN mkdir /app
WORKDIR /app
# install hex + rebar
RUN mix local.hex --force && \
mix local.rebar --force
# set build ENV
ENV MIX_ENV=prod
# install mix dependencies
COPY mix.exs mix.lock ./
RUN mix deps.get
RUN mix deps.compile
COPY config config
# build assets
# COPY assets assets
# RUN cd assets && npm install && npm run deploy
# RUN mix phx.digest
# build project
COPY priv priv
COPY lib lib
RUN mix compile
# build release
COPY rel rel
RUN mix release
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NobbZ
Compile time configuration is the magic word.
tao
The first command to not use the cache is the first COPY:
remote: Sending build context to Docker daemon 162.3kB
remote:
remote:
remote: Step 1/26 : FROM elixir:1.9.0-alpine as build
remote: ---> 7a6d28e4b511
remote: Step 2/26 : RUN apk add --update git build-base
remote: ---> Using cache
remote: ---> 11da61788331
remote: Step 3/26 : RUN mkdir /app
remote: ---> Using cache
remote: ---> 2a0fd2bc9f5c
remote: Step 4/26 : WORKDIR /app
remote: ---> Using cache
remote: ---> 876adb368066
remote: Step 5/26 : RUN mix local.hex --force && mix local.rebar --force
remote: ---> Using cache
remote: ---> eb75832955a0
remote: Step 6/26 : ENV MIX_ENV=prod
remote: ---> Using cache
remote: ---> 3bc485b4dc82
remote: Step 7/26 : COPY mix.exs mix.lock ./
remote: ---> 223512ca1219
remote: Step 8/26 : COPY config config
remote: ---> 09c813490fe9
<snip>
I’m just separating it into two commands like you suggested now. I’ll update here when I try this out! FWIW this is running on Dokku, so caching should be supported.
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