Demonstrate layer caching behaviour with non-deterministic commands. We pull the time from a time service to demonstrate the non-determinism. This would be akin to any command requiring an updated file.
Build the container without caching. The output of the build will show a build with a time pulled from an echo service.
docker build -f makecurl.Dockerfile --no-cache -t 16_cache_fails .
...
abbreviation: GMT
client_ip: 213.205.194.90
datetime: 2020-01-14T09:20:50.034710+00:00
day_of_week: 2
day_of_year: 14
dst: false
dst_from:
dst_offset: 0
dst_until:
raw_offset: 0
timezone: Europe/London
unixtime: 1578993650
utc_datetime: 2020-01-14T09:20:50.034710+00:00
utc_offset: +00:00
week_number: 3
...Running the image should print out the time the file was pulled.
docker run -it --rm 16_cache_fails
abbreviation: GMT
client_ip: 213.205.194.90
datetime: 2020-01-14T09:20:50.034710+00:00
...Now build again and the image should be fully cached.
docker build -f makecurl.Dockerfile -t 16_cache_fails .
Sending build context to Docker daemon 5.12kB
Step 1/7 : FROM ubuntu:latest as base
---> 549b9b86cb8d
Step 2/7 : RUN apt update && apt install make curl -y
---> Using cache
---> 8b1839a772da
Step 3/7 : RUN echo "Hello"
...Therefore, the image is not rebuilt and we have an old file.
docker run -it --rm 16_cache_failsNo caching will occur here as the order of the commands in the apt are different.
This means the subsequent layers are also invalidated and everything is rebuilt.
docker build -f curlmake.Dockerfile -t 16_cache_fails .