Docker build files and container network connections are controlled by separate boundaries. A build step can only use files made available to the build through its context or another declared source; changing its network mode does not grant access to more host files. At runtime, a bridge-network container can commonly make outbound connections through masquerading, while inbound access requires explicit port publishing or routing.
Why a Dockerfile cannot see a neighboring file
Docker Docs defines the build context as “the set of files that your build can access.” For a local build, the positional path in the build command selects that context. The Dockerfile’s location does not grant the build permission to read arbitrary neighboring host directories.
For example, in docker build -f app/Dockerfile ., the final . is the context: the current directory. A COPY source is resolved within that context. Using ../something does not let it escape the context. Docker’s documentation explains both the context boundary and Dockerfile source paths: Build context and Dockerfile reference.
Check the context and ignore rules first
- Inspect the path at the end of the build command; it determines which local files are supplied.
- Check whether a
.dockerignorefile excludes the missing file or its directory. - If a file lives elsewhere, deliberately make that directory available as a named context rather than trying to traverse upward with
...
For example, a named context can be supplied with --build-context shared=../shared and then referenced deliberately in the Dockerfile. Docker documents named contexts and their use in build instructions at Build context.
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A remote Git repository or tarball can also supply a context. By contrast, a text-only Dockerfile passed with docker build - has no local filesystem context, so it cannot copy files from the host.
How to use a context file during a build
Choose based on whether the file should remain in the image or is needed only while a particular build command runs.
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| Method | Input source | Remains in image? | When it fits |
|---|---|---|---|
COPY or appropriate ADD |
Build context or another declared source | Yes, in the stage | The file is needed in the resulting image |
BuildKit bind mount for RUN |
A file made available to the build | No; the mount is temporary for that instruction | The command needs an input, but the image does not |
| Named context | A separately declared directory or supported source | Only if copied or otherwise used in a persistent image operation | The input is outside the default context |
| Another stage or image | A declared build stage or image | Depends on how it is used | The needed artifact was produced or stored elsewhere |
Persist the file with COPY
Use COPY when the file must be present in the built stage, such as application source or a runtime configuration file. Keep the context as narrow as practical: expanding it can make more files available to the build, including files you did not intend to include.
Make a file temporary with a BuildKit bind mount
For an input such as a dependency list that is needed only during installation, BuildKit’s RUN --mount=type=bind makes the file available to that instruction without persisting the mounted file in the resulting image:
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# syntax=docker/dockerfile:1
RUN --mount=type=bind,source=requirements.txt,target=/tmp/requirements.txt
pip install --requirement /tmp/requirements.txtHere, requirements.txt must still be available to the build; the mount changes how it is presented to that RUN, not which host files the builder can access. Docker describes this behavior in Build best practices.
Does docker build --network=host fix file access?
No. Build networking and build-context access solve different problems. A network option can affect whether a build command connects to a package server; it cannot make an excluded or out-of-context file readable.
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BuildKit supports default, none, and host network modes for a RUN instruction. The default uses the builder’s normal networking, none isolates the instruction from network access while retaining loopback, and host uses the host network environment. BuildKit requires the network.host entitlement to be allowed by both the builder and the build request. See the Dockerfile reference and docker image build CLI reference.
- If a build command cannot download a package, check its
RUN --networkmode, DNS, proxy configuration, and builder environment. - If it cannot read a source file, check the selected context,
.dockerignore, and any declared mounts or named contexts.
Why a container can reach the internet but the internet cannot reach it
At runtime, Docker bridge networking commonly uses masquerading for outbound container connections. That lets a container initiate a connection through the host, but it does not automatically publish a service running inside the container to incoming connections. Docker handles incoming access separately: a published port maps a host address and port to a container port.
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To request that mapping, use -p when creating the container, for example docker run -p 8080:80 IMAGE. This maps host port 8080 to container port 80; it does not guarantee that the service is reachable from every external network. Host firewall rules, the host’s address and network, and other routing configuration can affect reachability. Docker explains port publishing and mapping at Port publishing and mapping.
Thus, “NAT only goes one way” is useful shorthand for the common bridge-network default, not a rule that inbound connections are impossible. Explicit port publishing or suitable routing can provide inbound access.
Diagnose the boundary that is failing
| Symptom | Boundary to inspect | First checks |
|---|---|---|
COPY cannot find a file, or RUN cannot read it |
Build filesystem access | Build command context path, .dockerignore, named contexts, and mounts |
| A build command cannot fetch a package | Build-time networking | RUN --network, DNS, proxy, and builder configuration |
| A running container can connect out, but external clients cannot connect in | Runtime inbound networking | Port publication, host address, firewall, and routing |
Changing a network flag cannot expose host files to a build, and mounting a file cannot create network reachability. Treat file access, build-time downloads, and runtime inbound connections as three distinct checks.
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