How to fix failed to solve: failed to compute cache key: failed to calculate checksum: no build stage in current context in Docker

Check that your Dockerfile starts with a FROM instruction (ignoring comments). The first non-comment line must be FROM.

Incorrect:

ENV SERVER_PORT=8080
FROM python:3.7-alpine

Correct:

FROM python:3.7-alpine
ENV SERVER_PORT=8080

If you need to use variables before FROM, use ARG (the only instruction allowed before FROM):

ARG BASE_IMAGE=python:3.7-alpine
FROM ${BASE_IMAGE}
ENV SERVER_PORT=8080

Move all instructions (ENV, RUN, LABEL, MAINTAINER, etc.) to after the FROM statement. Only ARG instructions are permitted before FROM.

Check for these common mistakes:
- MAINTAINER instruction before FROM (deprecated anyway, use LABEL instead)
- ENV declarations before FROM
- RUN commands before FROM
- LABEL instructions before FROM

# WRONG
MAINTAINER John Doe <[email protected]>
FROM ubuntu:20.04

# CORRECT
FROM ubuntu:20.04
LABEL maintainer="John Doe <[email protected]>"

If using multi-stage builds, ensure each stage is properly named and referenced:

Check stage naming:

# Build Stage - use AS to name the stage
FROM maven:3.8.1-jdk-11 AS builder
WORKDIR /build
COPY . .
RUN mvn package

# Final Stage
FROM openjdk:11-jre
# Reference must match exactly
COPY --from=builder /build/target/app.jar /app.jar
CMD ["java", "-jar", "/app.jar"]

Common mistakes:
- Typo in stage name: COPY --from=build when stage is named builder
- Missing AS clause: FROM maven:3.8.1-jdk-11 without a name
- Case sensitivity: COPY --from=Builder vs AS builder

If you're building a minimal image without a base image, you must explicitly declare this:

FROM scratch
COPY myapp /
CMD ["/myapp"]

Don't omit the FROM instruction entirely, even for source images.

Hidden or non-printable characters can cause parsing errors. Try these steps:

1. View raw file contents:

cat -A Dockerfile

Look for ^M (Windows line endings) or other unusual characters.

2. Convert line endings if needed:

dos2unix Dockerfile
# or
sed -i 's/\r$//' Dockerfile

3. Recreate the Dockerfile:
If suspicious, copy the content to a new file using a plain text editor to eliminate hidden characters.

Ensure your Dockerfile is in the build context and not excluded:

1. Run build from correct directory:

# If Dockerfile is in project root
cd /path/to/project
docker build -t myapp .

# If Dockerfile is elsewhere
docker build -t myapp -f path/to/Dockerfile .

2. Check .dockerignore file:

cat .dockerignore

Make sure these patterns are NOT in .dockerignore:
- Dockerfile
- Dockerfile*
- * (excluding everything)

If the error appeared after enabling BuildKit, test with the legacy builder to isolate the issue:

# Disable BuildKit for one build
DOCKER_BUILDKIT=0 docker build -t myapp .

If this works, the issue is BuildKit-specific. BuildKit is stricter about Dockerfile syntax. Review your Dockerfile for:
- Proper stage naming in multi-stage builds
- Correct instruction ordering
- Valid syntax according to Dockerfile reference

Note: While the legacy builder may work, it's better to fix the Dockerfile for BuildKit compatibility as the legacy builder is deprecated.