Understanding Docker's Build Cache Mechanism

Docker's caching mechanism significantly optimizes image builds by reusing unchanged layers, which reduces storage requirements and build times. However, effectively utilizing this cache requires understanding its behavior and limitations.

Cache Behavior Considerations

1. ADD and COPY Commands: Even when the Dockerfile remains unchanged, modifications to source files referenced in these commands should invalidate the cache. Docker determines file changes by calculating a hash of the file contents and metadata. If this hash differs from the previous build, Docker considers the file modified and rebuilds the layer.
2. External Dependencies in RUN Commands: Commands like RUN apt-get update introduce external dependencies that change over time. The same command executed months apart may produce different results due to updated package repositories. Docker provides the --no-cache parameter to handle such cases: docker build --no-cache -t="my_application" .
3. Layer Dependency Chain: Docker's layer hierarchy means that rebuilding any layer invalidates the cache for all subsequent layers. This is why Dockerfiles should place stable, infrequently changing commands (like dependency installation) near the beginning and frequently chenging commands (like application code copying) near the end.

When building images with a Dockerfile, Docker leverages its cache by only rebuilding layers whose corresponding commands have changed. Consider this example of building a web application:

$ docker build .
Sending build context to Docker daemon  2.56 kB
Step 1/7 : FROM node:14
  ---> d76c7e8bd352
Step 2/7 : MAINTAINER developer@example.com
 ---> Using cache
 ---> 8f5a8a3d9240
Step 3/7 : RUN git clone -q https://github.com/example/webapp.git
 ---> Using cache
 ---> 48db97331aa2
Step 4/7 : WORKDIR webapp
 ---> Using cache
 ---> c5c85db751d6
Step 5/7 : RUN npm install --silent
 ---> Using cache
 ---> be943c45c55b
Step 6/7 : EXPOSE 3000
 ---> Using cache
 ---> 805b18d28a65
Step 7/7 : CMD ["npm", "start"]
 ---> Using cache
 ---> 19525d4ec794
Successfully built 19525d4ec794

While caching is beneficial for performance, it can sometimes lead to unexpected behavior. Using the above Dockerfile as an example, if you update the application code in the Git repository, the new code won't be pulled during subsequent builds. Docker sees the git clone command as unchanged and reuses the cached layer instead of executing the command again.

Problem

You need to rebuild a Docker image from scratch, bypassing Docker's cache mechanism to ensure all layers are freshly created.

Solution

Use the --no-cache parameter when executing the docker build command to force Docker to rebuild every layer without utilizing any cached layers.

Implementation

To disable caching during the build process, append the --no-cache flag to your build command. The following example demonstrates building the same image without cache:

$ docker build --no-cache .
Sending build context to Docker daemon  2.56 kB
Step 1/7 : FROM node:14
  ---> d76c7e8bd352
Step 2/7 : MAINTAINER developer@example.com
 ---> Running in ca243b77f6a1
 ---> 602f1294d7f1
Removing intermediate container ca243b77f6a1
Step 3/7 : RUN git clone -q https://github.com/example/webapp.git
 ---> Running in f2c0ac021247
 ---> 04ee24faaf18
Removing intermediate container f2c0ac021247
Step 4/7 : WORKDIR webapp
 ---> Running in c2d9cd32c182
 ---> 4e0029de9074
Removing intermediate container c2d9cd32c182
Step 5/7 : RUN npm install --silent
 ---> Running in 79122dbf9e52
npm WARN package.json webapp@1.0.0 No repository field.
 ---> 9b6531f2036a
Removing intermediate container 79122dbf9e52
Step 6/7 : EXPOSE 3000
 ---> Running in d1d58e1c4b15
 ---> f7c1b9151108
Removing intermediate container d1d58e1c4b15
Step 7/7 : CMD ["npm", "start"]
 ---> Running in 697713ebb185
 ---> 74f9ad384859
Removing intermediate container 697713ebb185
Successfully built 74f9ad384859

Notice that in this build, Docker executes each step without using cached layers, resulting in different layer IDs for each step compared to the cached build.