A repository with history and dependencies
Both uploaded on every build, and neither is needed in the image
200M ./.git 50M ./node_modules 5M ./src 1M ./package.json
Docker Build Context Analyzer
Find out what the pause labelled sending build context is actually sending. Paste the output of du -a . or find . -type f -printf '%s %p\n'.
Paste below, or drop a file anywhere on this panel
Or drop a file anywhere on this panel. Nothing is uploaded: the analysis runs in this tab.
The answer appears here
Paste on the left and press Analyze. Nothing leaves this tab.
Formatted
Results
Nothing else to flag.
No formatting problems, and nothing the rules object to. Worth remembering what that covers: this reads the file you pasted, not the account or cluster it will be applied to.
No finding matches that filter.
Examples
Real input you can load into the tool above. Each one shows a different thing going wrong, because that is what the tool is for.
A clean context
Still uploaded in full before the first instruction runs
4M ./src/app.js 1M ./src/vendor.js 1M ./package.json
Common mistakes
These are the ones that fail silently. The config is accepted, nothing raises an error, and the consequence arrives later.
Leaving .git out of .dockerignore
The entire repository history is uploaded on every build and is never used. It is usually the largest thing in the context.
Instead:Add .git. It is the single highest-value line in that file.
Blaming a slow build on the network or the registry
The pause before the first instruction is the context upload, which happens locally and has nothing to do with either.
Instead:Read what the context actually contains before optimising anything else.
Building from a subdirectory to shrink the context
The context is the path you pass, not the Dockerfile's location, so this changes nothing.
Instead:Use .dockerignore, which is the only lever.
The whole context is transferred before the first instruction runs
Not the files a COPY needs: everything you passed as the build context. It happens on every build whether or not any of it is used.
.git is usually the largest single thing
The whole repository history, uploaded every build, and never needed inside the image. On a repository with any age it dominates the pause before anything appears to happen, and a single .dockerignore line removes it.
node_modules is slow and wrong
Slow to upload, and wrong to copy: a local install may hold binaries built for a different platform than the image, which then fail at run time with an architecture error rather than an obvious one. The install belongs inside the build, where it also caches properly.
BuildKit makes the second build cheaper, not the first
It transfers incrementally and caches between builds, so the full cost is paid once per builder and then only for what changed. On an ephemeral CI runner that means every job pays it in full.
A remote daemon pays more
Docker Desktop puts the daemon behind a VM boundary and a remote daemon puts it across a network, so the same context costs considerably more than it does with a local daemon on Linux.
Building from a subdirectory does not help
The context is what you pass on the command line, not what the Dockerfile reads. docker build -f sub/Dockerfile . still sends the whole current directory.
A .dockerignore is read before anything is sent
So the excluded files never leave your machine. It also stabilises the build cache, because a file that is not in the context cannot invalidate a COPY layer by changing.
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