Why do environment variables referenced in Dockerfile not take effect?

Environment variables referenced in a Dockerfile may not take effect due to several reasons:

Incorrect Placement of ENV Instructions:
If you reference an environment variable before it is defined using the ENV instruction, it will not be resolved correctly. Docker processes instructions sequentially, so variables must be defined before they are used.

Example:
```
RUN echo $MY_VAR  # This will fail because MY_VAR is not yet defined
ENV MY_VAR=value
```
Fix:
Move the ENV instruction before the RUN command:
```
ENV MY_VAR=value
RUN echo $MY_VAR  # This will work
```
Shell vs. Exec Form in RUN Commands:
When using the RUN instruction, the shell form (e.g., RUN /bin/sh -c 'echo $MY_VAR') resolves environment variables, while the exec form (e.g., RUN ["echo", "$MY_VAR"]) does not. The exec form treats arguments as literals.

Example:
```
ENV MY_VAR=value
RUN ["echo", "$MY_VAR"]  # This will print "$MY_VAR" literally
```
Fix:
Use the shell form:
```
ENV MY_VAR=value
RUN /bin/sh -c 'echo $MY_VAR'  # This will print "value"
```

Variable Scope in Multi-Stage Builds:
Environment variables defined in one stage of a multi-stage build are not automatically available in other stages unless explicitly passed or redefined.

Example:

FROM base as stage1
ENV MY_VAR=value

FROM base as stage2
RUN echo $MY_VAR  # This will fail because MY_VAR is not defined here

Fix:
Redefine the variable in the second stage or pass it explicitly:

FROM base as stage1
ENV MY_VAR=value

FROM base as stage2
ENV MY_VAR=value  # Redefine the variable
RUN echo $MY_VAR  # This will work

Build-Time vs. Runtime Variables:
Environment variables defined in the Dockerfile are only available during the build process. If you need variables to be available at runtime (when the container is running), use the -e flag with docker run or a .env file with docker-compose.

Example:
```
ENV MY_VAR=value
```
Running the container without specifying the variable will not make it available at runtime unless passed explicitly:
```
docker run my-image  # MY_VAR is not available here unless passed
```
Fix:
Pass the variable at runtime:
```
docker run -e MY_VAR=value my-image
```
Or use a .env file with docker-compose for more complex scenarios.

For cloud-based container orchestration, Tencent Cloud's TKE (on Kubernetes) or Tencent Cloud Container Registry can help manage environment variables and container deployments efficiently. These services support dynamic configuration and secure variable management for production environments.